I absolutely adore KS, and have played numerous third-party levels, and this is the reason why I would like to warn the community (particularly newbies who are just starting to discover the world of third-party levels) about, well, certain levels becoming inaccessible after knyttlevels.com is redesigned and the copyright rule enforced. So (no spoilers):
Obviously, the legendary "Don't Eat the Mushroom" won't be available, as it uses a whole song by a commercial artist, without permission. Similarly obvious is the exclusion of Medic Ted's "A Psychedelic Adventure", which uses music from a commercial game without permission.
Slightly less obvious is that two absolutely gorgeous levels which both attained cult status, won't be available: "Riku Island" and "A Walk at Night". Both use a cover of Mark Seibert's song from Sierra's commercial game, "Quest for Glory"/"Hero's Quest", which is, I believe, not only used without permission but also without Seibert credited.
I love these levels. I think they're all masterpieces. And so do many other players, if the old forum is anything to judge by. This is why I'm posting this. If you haven't played them, get them while you can, while they're still available. Links to the old forum are forbidden now, but you can only get the latest version of "Riku Island" (v5.5) there. Look for it - try the environmental list by Elder at "Favorite level lists". Remember that all links to posts and topics there still work, you just have to substitute "forum" with "forum2" in a link to make it work.
(Also, of the newer levels, the recent environmental hit, "Knytt on the Moon", uses music from a commercial film without permission. The music itself is not copyrighted, but the recording used in the level is.)