Update: Uh, apparently GOG's DRM-Free Summer Sale (note: everything on GOG is DRM-Free) has not yet ended. Those deals are definitely worth your time to check out.
While I missed writing about GOG's Summer Sale (okay, at the time of this publishing, theres a few hours left), I can at least alert our readers about these two deals. Splinter Cell: Blacklist, released late last summer, is almost two-thirds off at $10.19 USD (normally $29.99). Also on sale is Thief, released in late February, for about half price at $14.99 USD (normally $29.99). If you purchased these games and wanted a little more content to them, each title's downloadable content is on sale for an equivalent markdown.
Image Credit: Klow from CombineOverwiki
While not as unforgiving as Splinter Cell: Conviction, a UPLAY account is required to activate Blacklist. You do not, however, need to be continuously logged in to it in order to play its single player mode. I believe that Thief, on the other hand, just uses Steam for its DRM.
Splinter cell is going on
Splinter cell is going on sale on GOG soon as a flash sale
Classic Thief games are on
Classic Thief games are on sale there too. http://www.gog.com/promo/summer_thief_bundle_150614