The preview beta for Good Old Games started this week, and after combing through their list of available (or soon to be available) titles, we have discovered several classic PC titles with co-op modes! All games on the service are sold DRM-free at very reasonable prices, and have been updated to work with XP and Vista.
Co-Op Titles:
Other Titles of Note:
Pretty much every title on this list will set you back $5.99, so you can get a lot of bang for your gaming buck.