One batch of my favorites. Don Cherry had a series of great covers in the late 60s through the 70s. They'll obviously not be everyone's taste, but they're interestingly quirky at least. They were all done by Moki (sometimes spelled "Moqui"), Don Cherry's wife. There are better pictures of some of these online (discogs is often a good source), with more accurate colors, etc., but I couldn't always find good pictures to link to. They're great albums, too, by the way.
1969 - Where Is Brooklyn?
1970 - Human Music
1971 - Organic Music Society
1973 - Relativity Suite
1974 - Eternal Now
1975 - Brown Rice
1977 - Hear & Now
1978 - Live in Ankara
Cherry had some great covers aside from these, too, but I'll keep this post to the ones Moki did.