Thought up a few more in the case that namida decides to add more skills:
Corroder - a permanent skill where lemmings can destroy any steel. This might cause potential backroutes, though (like what happened with ghosts' ability to bash one-way walls before namida removed that).
Ramper (suggested by bsmith in the Fencer skill topic) - builders can build downwards. Since we already have horizontal/diagonal-up construction skills, I don't see why we can't have one diagonal-down one as well.
Armorer (feel free to come up with a better name!) - a permanent skill which turns the lemming invincible - they can walk through water and continuous traps (i.e. fire), treat floors/walls as solid, and walk past traps (but not disarm them!). This is especially useful for continuous traps as we have yet to have a skill that bypasses these (like the disarmer for regular traps and the swimmer for water).
Remover (suggested by möbius in his topic) - removes a permanent skill from a lemming. You may choose which permanent skill to remove.