I had an old rigid MTB that I rebuilt into a commuter after the frame cracked (too many good parts on it to waste). I used this fork on it to also get a little more height in the front end:

Nashbar Rigid 26" Mountain Bike Fork

It seems to be fabricated to substitute for 100mm travel suspension forks, and makes the front end 'floppy' when going really slow (it changes the 'trail' of the fork/wheel setup), but feels OK when up to normal road speed; I still wouldn't ride it 'no hands' at speed as it does have a little flop to it, but I'm happy with the final build. I wouldn't use ti exclusively on trails, but a dirt path wouldn't be a problem.