I used CreateSpace (Amazon). I discuss it a bit here: http://bitmason.blogspot.com/2013/02/decisions-i-made-when-p... I wanted a "regular" book including a printed version--it's effectively supporting material for my day job and I needed a physical, as well as an electronic, version. It was all a bit of a learning process but I'm happy with the result. And Amazon's a good platform to be on.

Leanpub's an interesting-looking concept but I'm not especially sold on the value. Why would someone want to buy an unfinished manuscript? If you want to flesh out some ideas, that's probably better done through discussion forums and blog posts before assembling into a book.