I use iA Writer Duo for writing prose and fiction a lot.

If you're willing to pay for typefaces, though, there's a really good "almost typewriter" font called Triplicate, which comes in four versions -- the original, a slight variation with the same metrics as Courier, a "code" version that has a few characters changed to be better for coding (this is a great alternative to the more expensive Operator Mono, in my experience), and a proportional one which fits the "almost monospaced" idea. (It's not designed for coding, but I don't think proportional fonts work for coding in most editors.)