If you're into shooters, I can personally reccommend Nexuiz [http://www.alientrap.org/nexuiz/] and Quake Live [http://www.quakelive.com/#home]. Wikipedia [http://en.wikipedia.org/wiki/Free_First_Person_Shooters] has an exhaustive list as well.
If you're into Elite like space sims, there's Privateer Gemini Gold [http://privateer.sourceforge.net/], the other games based on the same engine, and of course, Elite [http://www.iancgbell.clara.net/elite/bbc/index.htm] itself. There's also Frontier: Elite II and Frontier: First Encounters, both of which can be found here [http://www.hooplatec.com/encounters/files.htm]. Note for mods: I know the last link wasn't to an official page, but it was just because I couldn't find it. These are mirrors of a shareware version, but like Winrar, the only thing making them shareware is a nag screen at the beginning of the game. They are definitely not Warez.
For RPG's, there's Betrayal at Krondor [http://compactiongames.about.com/od/freegames09ac/p/krondor_free.htm], as well as the first two Elder Scrolls games, both of which are available here [http://www.elderscrolls.com/downloads/downloads_games.htm]. Daggerfall in particular is awesome, and there's an engine rebuild called Dagger XL [http://daggerxl.wordpress.com/] that should be playable by the end of the year. Daggerfall makes both Oblivion and Morrowind look like rail shooters by comparison, it's truly a go anywhere do anything game. With DaggerXL, Lucius (the person writing the program) plans on creating a modding platform for the game, and making it possible for all the features that had to be cut for time and performance restraints to be re-implemented. The terrain system, for example, was already heavily improved by the first pre-alpha build.
For Point and Clicks, there's the VGA remakes of King's Quest I and II, as well as Quest For Glory II, all of which are available here [http://www.agdinteractive.com/games/games.html]. There's also quite a few freeware point and click's on the SCUMM VM downloads page [http://scummvm.org/downloads/].
Note: For many of these games, you'll need a copy of dosbox [http://www.dosbox.com/] to get them running. For Frontier: First Encounters, You'll want a copy of JJFFE [http://jaj22.org.uk/jjffe/] instead. For the games on the SCUMM VM site, obviously, you can get SCUMM VM from the same page to run them.