Vagabond: Only a few,
here's the supported game list.
Even among those few that are "supported" there are some that do not work - Abe's Odyssee, Abe's Exodus, and Broken Sword: The Hidden Dragon are ones I know of. Xfire only support the Steam version of the Abe games, and the GOG.com version of Broken Sword 3 have a different name of its game exe than what Xfire is looking for.
Both of these cases can be changed with little work (by editing xfire_games.ini, somewhere on your hard drive, different locations for XP and Vista/Windows 7) so that Xfire correctly can detect them as installed and start them.
There may be others in the same situation, then there are those that already work perfectly (examples: TOCA 3, and apparenly Postal 2), those that work kinda-sorta (ex: Beyond Good and Evil, Xfire doesn't run GOG.com's special launcher, otherwise it's fine), and those that can be added but won't track played time or be seen by others when you're playing them (ex: MDK2, and games running on DOSbox or ScummVM).
I have made some additions to my Xfire.ini to check for those game I have (by the way, this thread is a duplicate, though I cannot be bothered to look for and link to the older one... Weclock, didn't you do that for a while, point out duplicates?)
I'll post the link to my file when I have waded through it for obvious errors... and reinstalled my games as I upgraded to Windows 7 a couple of days ago.