You might be interested in Lix [1], it's like Lemmings but with networked multiplayer, and it's open source (CC0).

[1] http://asdfasdf.ethz.ch/~simon/about.php

There is a Lemmings clone in DHTML but it seems to have legal problems [1].

http://crisp.home.xs4all.nl/lemmings/lemmings.html

I was waiting for that one to pop up :p. That is actually a web game from way before web games, AJAX, or Javascript animations were a thing. That's probably just as important history as the Lemmings game itself is.