Typically? Crap. The site says so itself. Generally it comes with a hodge podge of things you generally wouldn't buy, though at the really low price of $1.
To be fair, every bag of crap comes with
1) at least one bag
2) crap
You can only buy up to three of anything on Woot, and it goes for bags of crap as well. So generally your total will hopefully go to $8 ($5 shipping is standard), assuming you get three bags of crap. If you are lucky enough to select the 'I want 3' option and come away with three bags of crap, generally they'll give you more than three craps, but not always more than one bag. Sometimes they'll even pack in bags of Texas air!!!
For the record, in my last (and first) three bags of crap, I received:
1) 1x An ipod pillow that you can hook your ipod up to on the inside and it'll play through a speaker inside of the pillow (not that comfortable sadly)
2) 3x 6-slot wine carrying bag
3) 2x 1-slot wine carrying bag
4) 1x 45 gramophone record of... something along the lines of 'My First Classical Composers' songs
5) 2x Electronic Sudoku handheld games
6) 0x bags of Texas air... I guess they assumed that, living in Austin, TX, I already had my own Texas air
Generally they sell out within 30 seconds of being posted though, and Woot servers are easily bogged, so unless you're really looking for it, you're plum out of luck.