Android: We've had a premium Album Art Grabber in the Market for awhile, but Tim Clark's new Album Art Grabber app is not only free, it's better, featuring an easy-to-use interface and searching from both Last.fm and MusicBrainz.
The app is pretty self-explanatory: just start it up and it will present you with a list of all the albums on your SD card. You can hit the "Grab!" button to automatically download all the missing art in your collection, as well as tap individual albums to pick the art yourself. It will give you the choice of searching Last.fm or MusicBrainz and let you pick which cover you like best from the results.
I've found that not only is the interface a bit cleaner than the premium grabber, but it consistently finds better art (mostly through Last.fm). So if your library is still dull and grey, you have no excuse not to fire this up and get it looking snazzy. Note that this doesn't actually edit the ID3 tags, so if you add art to an album on your phone, it won't have that art if you transfer those songs back to your computer.
Album Art Grabber is a free download for Android devices.
Album Art Grabber [Blog Tim]