It seems many of Telltale's games are going on sale on other online game stores and websites, especially now that we're approaching the Christmas period. So here are just a few.
Back to the Future is currently down to $21.25 on Direct2Drive, and you'll be able to save even more seeing with the '20% off all orders' deal/coupon.
Wallace and Gromit's Grand Adventures - Being cut down by 66%, this game is now only $6.78; that's less than a single episode, normally. Plus, since it's available on GamersGate, you'll be awarded blue coins for buying it (as well as for rating it and reviewing it), which can be spent on future purchases from the site.
Puzzle Agent - It's currently half price and down to just $5 from GamersGate. Also, it includes both the Mac and PC versions.
That's all I could find for the time being. If you come across any more, then post them here; that's what this thread is for.