Wyrmkeep has made Linux versions of Inherit the Earth and Labyrinth of Time
http://www.wyrmkeep.com/Many of the Wadget Eye games can be played on Linux, but they seem to require configuration because they're not exactly "Linux versions." Here's a thread where this is discussed:
http://www.wadjeteyegames.com/forum/index.php?topic=1508.0Phoenix Online offers Linux versions of a couple of their games if you buy through their website
(
***The Last Door***,
***Quest for Infamy***). Quest for Infamy is more of a hybrid adventure/RPG though, the humor may be a problem, and it isn't even available yet.
Many older games can be played in ScummVM, and there are some freeware versions that can be downloaded from ScummVM. Games like "Beneath a Steel Sky" and "Flight of the Amazon Queen" may even install by default on some versions of Linux. There are a few other freeware games that can be downloaded from the Scumm website
http://scummvm.org/games/ScummVM now supports many games besides LucasArts games, including many Sierra games. It does not matter if you're using the Linux version of ScummVM as opposed to a PC or Mac version, and the list of games includes some Windows games -- not just DOS games.
http://scummvm.org/compatibility/Many older PC games will work in WINE, however it's not as easy to use as ScummVM. Depending on the expertise of your friends, they may want to stick with ScummVM to start out with.
If you are interested in Humble Bundle, the Wikipedia has an article on it
http://en.wikipedia.org/wiki/Humble_BundleI tend not to trust things where I can't understand the purchasing system, and words like "unlock" in the purchase description don't sound DRM-free to me.
Also, when something claims to be "DRM-free" but requires some sort of client, it isn't truly DRM-free in my opinion. YMMV
I'm glad to see ShinyLoot is offering some adventure games for Linux, like The Raven and The Journey Down. Note that they have two versions of The Raven -- the regular version, and the Collector's (for $5 more).