http://steamdb.info tracks current and historic Steam sales (among other data). It’s somewhat obtuse because Steam’s back-end is, but in the simplest use, look up a game and scroll down to see current / historic low price in your local currency.
These days, almost every historic discount repeats in the big Steam sales, so ’cheaper than Steam’s low price’ is another rubric for what I’m hoping to see. Or the return of a discount that’s been absent for more than a year. Lately, the third-party key sites (the legit ones, like GMG) will one-up each other by doing 1 - 3% cheaper than the Steam low.
As long as we’re mentioning the usual suspects http://fanatical.com still often has cheapskate bundles of games you might want to play. I’ll try to call them out specifically in the future because they’re about to change out.