I feel like writing today, so I'll recap on all the important Lemmings clones/variants I know that actually use lems and not some other character.
Custlemm and Lemmix editor require graphics from the original game. They don't come with copyright-restricted files themselves. No legal problems, makes the applications just harder to setup.
Lemmix player comes with everything included. This is not 100 percent legally clean, but fares very fine so far. As fine as any abandonware site that hosts L1. Lemmix (both the player and the editor) is not that well known outside of level editing communities, I think. Note that there is no trademark infringement here, unlike with Lemmings for Iphone.
Lemmings DS by Matt continues to live. Forum activity is pretty low and there is little development these days. Matt works for Sony and they probably know that he made the game and hosts it on his site.
Palm Lemmings by Ardiri in 2001 didn't get blasted, and is still up for sale. This has amazed me for years. In an email to me, Matt from Lemmings DS thought the same. I believe the long life of Palm Lemmings was a major incline for Ardiri to port it to Iphone and Palm Pre, and to assume it had a fair chance of not getting axed either.
Cheapo from 2002 got axed and hasn't been developed further since. Its author said he lost the Cheapo source, but wanted to make Dimwits instead, which didn't reach a releasable state.
DHTML Lemmings was taken down by its author's webhoster after a C&D to both the author and the hoster. The C&D didn't even get to the substance of the case: It claimed he used code from L1, but said nothing about graphics, whereas DHTMLL uses the L1 graphics, but no code. Its author wrote an inquiry to Sony if they were okay with him hosting the game before. If he hadn't done that, I believe the game would've lived on for quite some time on its authors webspace.
Lemmings SDL for PC and Pandora by miner49er isn't released yet. He's taking a real-life break from programming right now, according to an email exchange in May. Don't know when he gets back into development. It's hard to tell what will happen to LSDL after its broad release.
L++ is down until the lix is drawn. Gonna be alright.
-- Simon