In general, don’t buy things that aren’t Daily or Flash Deals until the last day, or they’re pretty much guaranteed to go on sale the next day and make you kick yourself. That’s how it works.
That said, there are some games that are heavily-discounted or already cheap, which are unlikely to go lower. Of these, some are must-haves:
–Dear Esther – US$2.49: If you buy nothing else during this Steam sale, buy this. A wonderful experience with no shooting or puzzles, just walking across a windswept Hebridean island, and discovering a non-linear narrative for yourself.
– Introversion Pack – US$4.99: Includes Uplink, Darwinia, and Defcon, as well as soundtracks for all three, some of the most interesting and innovative minimalist PC games of the last decade and a half. Every one of these is a must-have.
– Cart Life – US$1.99 – This game is not fun. That’s what’s so excellent about it. It’s a simulator in which you play a street vendor, dealing with the problems and drudgeries of the life, and it’s a must-have experience.
– Psychonauts – US$2.49: If for some reason you haven’t played this, and/or missed the Double Fine Humble Bundle a couple months back, get this now. It’s as charming and fun as ever.
– Time Gentlemen, Please & Ben There, Dan That – US$0.49: Two games, both great homages to LucasArts adventure games
– Amanita Bundle – US$4.99: Includes Samorost 2, Machinarium and its soundtrack and Botanicula, all wonderfully-illustrated adventure games.
– Overlord Pack – US$4.99: great RTS/action hybrid, with writing by Rhianna Pratchett. It’s immensely satisfying to sweep a wave of minions over an enemy.
– The Blackwell Legacy – US$1.99: The first of the Blackwell series of adventure games, which have a low-res style akin to Sierra adventure games, but a contemporary setting and contemporary sensibilities in terms of their storytelling and puzzle design.
– Sine Mora – US$2.49: Horizontal bullet-hell shooter. I’m not generally a fan of the genre, but reliable sources tell me this is one of the most amazing examples to come out in recent years.
Even if any of these games do go on sale, they’re cheap enough that you won’t save much more, so you might as well buy them at any time during the sale.