A good REPL is one thing I find it really hard to program without. In many situations, I find ad-hoc testing by typing expressions in to the REPL to see that the return what I expect (or sometimes more experimentally, to see what they return so as to better understand an API) preferable to formal unit testing.

It's the main thing I miss when programming in Haskell. GHCi doesn't quite measure up to Lisp (or Ruby, or Python) REPLs.