Vendetta Online, which originally released on April 20th, 2002 and is still going strong today.
Guild Software is hosting a couple of celebration events this weekend. The company has also announced that its massive 1.9 patch is coming later this year, followed by an even more massive expansion in 2013.
Vendetta is unique among MMOs for its twitch-based space combat and extensive trading, mining, and exploration gameplay. Large space battles, dynamic territory control, and player-generated content systems round out the feature set, all of which is coded and maintained by a four-man dev team. The single-universe game server is accessible from a number of different platforms that include Windows, Mac, and Linux operating systems. In March 2011, Vendetta became the first PC MMO to jump to mobile platforms, and it spent several weeks as the number one paid app on Android devices.
Check out the game's official website for more info, and don't forget to sign up for the trial while you're there. You can also read Massively's latest game impressions in Some Assembly Required.
[Source: Guild press release]