I guess it really isn't a tool itself, a material rather. However, sure do love MDF, medium density fiberboard now that I can get it regularly in smaller panels. I make a lot of jigs, fixtures, and even tools from the stuff.
One of the simplest "tool" usages is for a cutting board for PE. Yeah, it doesn't last as long as some of those exotic plastic or ceramic blocks, but when I cut a 3 X 3 inch piece of the stuff, I don't mind throwing it out when it gets scarred up and cut a new one. And, even that scar covered block is often still useful for a jig.
I used to use a lot of particle board for that kind of stuff, but the MDF is so much nicer I don't mind paying a few cents more for it.