Steamalerts.com is amazing tool to use, I just set a game at a price I'm willing to buy and it will let me know when it gets that low, so far saved some dough on a few games unlike last year where games I bought went even cheaper than I bought them
I'd recommend http://www.cheapshark.com. It checks deals from Steam, Amazon, Origin, GOG, GMG, Gamersgate, and several others and will update you when a game falls below a certain price. I haven't purchased any games on Steam this year as at least one other site has consistently beaten Steam on every major sale. And most games on GMG and Amazon use Steam anyways and will even get you those trading cards so if you are looking for a specific game just put a price alert on that site. I promise you that not a single deal on Steam this year has been the best deal for any game they have outside of a select few older games. I got Bioshock Infinite, Tomb Raider, DMC, and several other major games from other sites for $5 or more less than on Steam.