If you've never played Half-Life, Half-Life 2, and the HL2 Episodes, STOP BEING STUPID!
The sale is part of GMG's 7 Days of Deals and good until Friday. See here for the other deals from today.
These prices match the cheapest they've been on Steam and are typically at 75% off during sales, so if you can't scrounge up a few bucks, just check during the next sale.
EDIT: And forgot to mention you can get an extra 10% Off:
EDIT2: Apparently this 20% Off code should work: O3H7FE-D4NFO0-F0LIGK (thanks, Dan)