You can now digitally download older Blizzard titles. Once you sign up for their store (if you haven't already), you can plug in your existing serial number to download games you already own; or purchase games you don't yet own and download them right away.
Currently Warcraft 3 and Starcraft (plus their expansions) are available, with Diablo 2 coming soon. I've plugged in my WC3 and D2 codes, and may snag them again if the mood hits. Unfortunately, I seem to have thrown away my Starcraft box, and lost my text file with the relevant serial numbers, so I'll just wait on Starcraft 2 for my zerg fix.