There aren't many freeware games out there that have a huge level of popularity. The most popular ones, I'd say, are those that were originally commercial but get bigger once the developers decide they need to be freeware. Behind those are abandonware (which isn't technically freeware) and those few freeware games that are amazing simply for being developed as a free game with commercial-gaming production values. I'd say Tremulous falls within the last category.
This is a game so well done you'd swear it was once a commercial game. The ideas behind it are too original and well-executed to be commercial though, so really it's the only indicating factor that lets you know this game wasn't made by professionals.
In this online FPS/Strategy hybrid, you start as one of two groups: the Humans or the Aliens. Each group has its own specific characteristics, but their objectives are the same: win by building your own base and destroying theirs. Each team begins with a very basic, pre-built base that players can rearrange however they choose. Unlike other FPS/Strategy games, however, there is no "commander", so you basically have to form up your base and run your team as a sort of commune in which players fill needed roles (take THAT, Glenn Beck!).
As well, each team must employ a different set of tactics in order to win. Human teams tend to "camp" at their base more frequently than Aliens to prevent themselves from getting too thrashed and handing an advantage over to the opposition. The aliens, on the other hand, tear a page from the Starcraft book of "rush like heck". Often the best teams are those that work together and achieve their common goals by employing similar tactics and teaming up.
The team based gameplay can lead to problems, however, such as the appearances of "deconners" (players who sabotage a team by deconstructing a base for the other team), which are usually pretty uncommon as servers immediately ban such players without question while restarting the game. There's also the occasional disorganized mess of a team that doesn't cooperate or work together, but that usually only happens on bigger servers.
As I said before, the production values are professional. The game is based on the Q3 engine but still manages to deliver fantastic graphics with very well-made characters and settings, along with a generous helping of default and user-made levels. The interface, sounds, and gameplay are all more than satisfactory.
Although the game is professionally made and has excellent gameplay, it can (like all online games) get boring after awhile. Noobs and bad servers can bog it down a bit (as can lag). Still, the sheer production and originality of it warrant some gameplay. It's definitely cool to watch your base build up as you and your teammates go forth and kick major rear. A must-play for any freeware or FPS fan.