For their Good Old Games sale, I might finally pick up copies of Star Trek: Armada and Star Trek: Armada II, two games I thoroughly enjoyed when they came out and have some fond memories of. I keep forgetting GOG have them now, so perhaps it's time to make it so.
Are there any particular classics you're looking at? Perhaps some BattleZone or Baldur's Gate: Dark Alliance? Whatever you're in the mood for GOG have quite a good collection of classics up there now.
GOG has also started their Star Wars celebration sale for May 4th a little early, so if you're in the mood for some classic sci-fi that's another good chance.
For the free game you can grab a copy of Under The Moon, just scroll down the GOG homepage when logged in the see the giveaway banner.