Besides the heavy hitters from Valve that have been launched on Steam for Linux so far, a couple other smaller titles will arrive soon.
The firsts Half-Life had quite a few addons and mods, but the most successful of them were Opposing Force and Blue Shift.
In Opposing Force, players have the chance to play the role of soldier sent in to eliminate Gordon Freeman. It stands to reason that you will not be able to perform that task, but you will get to fight a lot of new aliens.
The protagonist of Blue-Shift is a lone security guard that tries to survive in the mayhem that follows the experiments from Black Mesa.
The other part of this information, about the Linux port, is taken from the CDR database and by no means is it completely certain. The CDR Database is a record of data that describes every Steam game and subscription provided by Valve for the Steam service.
Usually, the new entries in the Steam database are first visible in the CDR Database before their official launch.