Bastion Press -- an excellent publisher of d20 source books usable with Dungeons & Dragons -- is having a huge sale via its web site through Jan. 31. According to publisher Jim Butler:

All Bastion books are on sale at only $5.00 each with the exceptions of Oathbound: Domains of the Forge being $10.00 and with Arms & Armor v3.5, its companion e-Tools datasets, and the novel Forged

I'd been meaning to check out Bastion's stuff for a while -- their Torn Asunder critical hits source book for D&D was very crunchy, and I'd heard plenty of good stuff about their environment themed Into the Blue (water/ocean), Into the Black (dungeons/underground) and Into the Green (forests) via Mortality Radio.

This sale proved to be a great boon in that regard, and I picked up my own copy of Torn Asunder (and thus, I no longer need to sneak glances at Lance's), all three Into the... books, and Pale Designs: A Poisoner's Handbook.
Of these, my favorites are Torn Asunder and Pale Designs, which include lots of great rules for expanding your d20 game in new directions (particularly with Pale Designs -- I look forward to tormenting my players with unidentifiable -- to them -- poisons). The Into the... books are decent, but were much lighter, rules-wise. Still, for $5, they were well worth picking up.