Ever try tofu? (originally from Comestible Zone)
Believe it or not, it can be quite tasty if prepared correctly. I've had it where it tasted rather tasteless but I've also fixed it myself and it was quite good. Tofu can be quite good for you but it's got calories just like anything else. Give it a try sometime.
Tofu can take on the texture of what you're substituting it for.
Some tofu info and recipes.
Give it a try!