Many great games, DRM-free and pre-configured to work on modern most operating systems, so just buy, install and play. No need to do any compatibility tweaks, enter a 30 character key or do any online registration.
Yes, sometimes they are slightly more expensive than other digital distribution systems, but it's all DRM-free so up to you to decide how much premium that is worth.
$2.99
Enclave
Oddworld Abe's Exodus
Oddworld Abe's Oddysee
Alien Shooter 2 Reloaded
Time Commando
Dangerous Dave Pack
Fahrenheit (AKA Indigo Prophecy)
$4.99
Psychonauts
Carmageddon Max Pack
Unmechanical
Hotline Miami
Tiny and Big: Grandpa's Leftovers
$7.49
Retro City Rampage
Giana Sisters: Twisted Dreams
La Mulana
Personal recommendations:
Indigo Prophecy (from Quantic Dream, the people who brought you Heavy Rain and the upcoming Beyond: Two Souls) and Psychonauts (another masterpiece from Tim Schafer) are absolute steals at those prices.
Giana Sisters and La Mulana are also good, but they have been part of bigger sales in the past.
Unmechanical is a good little puzzle game. :)