There's a level editor for Lemmings 2:
http://aminet.net/package/game/edit/Lem2LevEd210Rg
Actually I think a list of games that have editors (either 'official' or third-party efforts like the one above and for other titles like Cannon Fodder) would be quite useful. There are plenty of games out there that would probably enjoy renewed interest if there were new levels developed, both commercial and public domain.