In 2007 we posted about where you could get free light bulbs from Pacific Gas & Electric (PG&E). Well, that offer was, as they say, for a limited time only, so I think you're out of luck.
In the meantime, though, PG&E is still providing great subsidies for those new-ish Compact Fluorescent (CFL) bulbs, like $2 for a 4-pack, which is way cheaper than the incandescent energy hogs. You might as well start unscrewing them and make the upgrade since the energy savings will have them pay for themselves.
They don't mention Walgreen's, but I'm pretty sure you can pick them up there too.