I’d particularly recommend 101 Encounters, for those times you are stuck (it’s also probably the most ‘generic’ thing on the site). But, if you’re looking for something very different, check out ‘The Earl’s Progress.’
Over on my site I have a few low-level D&D adventures. They are generic and easily dropped into most standard D&D campaigns. Naturally, they are free as well.
Well, there are always the free adventures on the official wotc site, which are mostly along the lines of expanded encounters, really.
There's also Blue Troll, but to be honest most of the stuff there is (a) for older editions and (b) of variable quality. You might find it better for idea mining than for useful fully playable adventures.