The "Digimon Tamers" series is very much a case of "take it as it comes." Everything that happens follows on from something that's happened before, in a nice, neat, orderly line that has no holes that need filling - there's really no need for a timeline for it. However, there are a great many things that happened in the past, to make the Tamers world what it is. And here is where you can read about them.
Note that any text in BLUE is factual, and actually happened in the real world. Or at least, in the case of things like SIGINT and Project Echelon, *allegedly* happened, as the government denies their existence.
The Atanasoff-Berry Computer (ABC) - the very first computer - is built, and lays down the foundations of the Digital World (from Brave Tamer video game, see next entry).
The first patented digital computer, ENIAC (Electronic and Numeric Integrator And Computer), is brought on-line. With it's activation, the foundations laid by the ABC are built upon, and the Digital World is created. From the ABC and ENIAC, various multiple Digital World realities grow, which run parallel with the various multiple Earth realities in existence in the multiverse.
This information comes from the "Brave Tamer" video game. While continuity glitches between the games and anime mean that the video games may or may not be in continuity, this game explains that all the myriad "alternate universes" in Digimon - "Digimon Adventure," "Digimon Tamers," "Digimon Frontier," etc - all grew out of ENIAC and the ABC. Hence, the ENIAC explanation can be taken as canon for this and all timelines.
The US and the UK make a secret agreement to start intercepting communications. They start work on a SIGNIT ("Signal Intelligence") project.
The very first computer virus, "the worm," infects the US Department of Defence computers. It is destroyed by the "reaper," a program designed and created to delete excess data and thereby control the growth of artificial intelligence in the world's computer network.
The US Department of Defense's SIGINT system, known as "Project Echelon," is brought on-line, with many communications bases in countries all over the world. It's theorised that the reaper program is the basis for this project's security system.
In various countries, attempts to create artificial intelligence using computers are underway. The result is the DigiGnomes, who grow from the matter of the digital network world created by the ABC and ENIAC.
Doctor Rob "Dolphin" McCoy begins to experiment in creating artificial life, using drawings made by his son as a basis for the creatures he designs. He is soon joined in his work by students Janyu "Tao" Wong, Goro "Shibumi" Mizuno, Rai "Curly" Aishuwarya, "Babel," and "Daisy" (real names not known). Together, they form the group called the Monster Makers - and create the beings known as Digimon.
Funding is withdrawn from the Monster Makers' project, and it is abandoned. However, the data of their digital creations accidentally finds it way into the digital plain created by the ABC and ENIAC, and there, they begin to live and grow. The "Digital World" is still small, but it grows with them.
1990 and onward
The Digital World exists as only one plain, where the early Digimon and Digi-Gnomes live - but they soon come under attack from the reaper program, which deletes many of them. However, it absorbs the data of those that it deletes, and then enters a period of inactivity, hiding itself away as it is mutated by the absorbed data, attaining sentience and increasing in power.
Meanwile, in the real world, Shibumi continues to work on the Digimon project on his own, determined to create a self-dependant life form that can exist and evolve on it's own. He writes an algorithm that will allow Digimon to evolve beyond their original specifications. With the reaper gone, Digimon are free to grow and evolve again.
After a car accident, Shibumi goes into a form of coma, which allows him to project his mind into the DigiWorld. At some undetermined point following this, Shibumi sends a Mokumon to Earth, where it assumes his form and carries out activities.
Four Digimon have evolved to the highest stage, and become the ruling "gods" of the Digital World - Azulongmon, Zhuqiaomon, Ebonwumon and Baihumon, the four Digimon Sovereign. The Digital World continues to grow, expanding into six plains, populated with small "miniverses" - localised areas specially created to suit the Digimon who inhabit them. Digimon continue to Digivolve, thanks to the power of the Shining Digivolution, a power source kept on the highest plain of the DigiWorld, where the Sovereign dwell, which provides the energy that enables Digimon to Digivolve.
Digimon becomes a children's virtual pet-type computer game, and immediately becomes a big hit.
E-mail interception by the US government begins. It is believed that a Project Echelon intercepticon facility is set up in Japan.
Digimon becomes a card game.
Public concern surrounding the invasions of privacy perpetrated by Project Echelon is raised. The US government denies it's existence.
Use of the network increases widely, and the Digital World grows as the amount of transmitted information increases.
The first season of the "Digimon" animated series is broadcast.
The second season of the "Digimon" animated series is broadcast.
The Japanese government sets up it's own SIGINT organisation to monitor the network, known as "Hypnos." The organisation soon becomes aware of the Digimon, existing autonomously within the network, and labels them "Wild Ones," setting up protocols to deal with them as they "Bio-Emerge" into the real world.
200X minus one
(note that "200X" is the officially-given year for the events of the "Digimon Tamers" series, meaning a year somewhere between 2001 and 2009)
Ryo defeats Rika in the Digimon card tournament of that year. Subsequently, he goes to the Digital World, and stays there, as his partner Digimon, Cyberdramon, is too feral to take into the real world safely.
The DigiGnomes use Shibumi's evolution algorithm to link humans with Digimon. The algorithm manifests itself in the real world via the cards of the Digimon game, making a "blue card."
While playing the newest Digimon video game, Henry Wong, son of Janyu Wong, comes to the conclusion that Digimon exist in a reality outside the game. As he expresses remorse for hurting the Digimon within the game, a blue card appears, transforming his card reader into a D-Power Digivice, and bringing his partner, Terriermon, to the real world.
Due to Ryo's absence, Rika claims victory at the year's Digimon card tournament. Following this, Rika is confronted by a multitude of Digimon, who all want her to tame them. She only wants one, strong Digimon, however, and a blue card appears before her, transforming her card reader into a D-Power Digivice, which brings her partner, Renamon, to the real world.
Meanwhile, in the Digital World, the reaper program is re-awakened by the sheer amount of excess data created by Digivolving. The four Sovereigns are forced to take action as the "D-Reaper" sets about deleting the Digital World, carrying out it's original programming with extreme prejudice - and the power to back it up. Azulongmon has the DigiGnomes transform the Shining Digivolution, to hide it, and do away with Digivolution all together, as it was what caused the problem in the first place. The Shining Digivolution becomes Calumon, who is then expelled from the Digital World, and winds up on Earth.
Following this, the events of the Digimon Tamers series occur.
This information is mainly adapted from the "Chronicle" on Chiaki Konaka's website, translated (poorly, I might add!) by Altavista/Babelfish - although since this timeline's construction, the Chronicle has become available in English on his site (although it's still a bit... wordy). I've also inserted information that was given in the show. Also, some information was derived from various sites I found while searching for more information on SIGINT and Project Echelon.