Lemmings Level Editors
Lemmix - Eric Langedijk / EricLang
Lemmix is a Lemmings level editor for Windows. It is currently in the alpha development stage, but already supports many level formats.
LemEdit - VTM
LemEdit was the first Lemmings level editor. It has been hacked by ccexplore and tseug and should work in DOSBox. The unhacked version will only run in pure DOS. VTM also made Customized Lemmings so that levelpacks made with LemEdit could be easily played.