http://store.steampowered.com/genre/Adventure
LucasArts Adventure Pack (4 games)
$2.49
Machinarium
$9.99
Indigo Prophecy
$3.39
Broken Sword 2
$2.99
Broken Sword 3
$2.99
Nikopol: Secrets of the Immortals
3.99
Time Gentlemen Please!
$2.49
Samorost 2
$2.49
Non-adventures:
The Witcher
$13.59 - fantastic RPG.
S.T.A.L.K.E.R.
$1.49
Mirror's Edge
$4.99
...to name a few.