Oleg Kiselyov's Iteratee. I still don't really understand how it works, but it's saved my life a couple of times when I've needed to do something like read 60,000 files quickly and didn't have time to figure out why the normal Haskell file access approaches were so slow.

Also Brian O'Sullivan's Attoparsec, which in my experience does perform much better than Parsec on things like large flat-file databases.

And now you can combine the two.

Conal Elliott's Vector-space packages up a lot of linear algebra functions very nicely, and also got me a little closer to feeling comfortable with type families.

Statistics (again by O'Sullivan) has been handy because I'd prefer to work in Haskell than in R.

HXT and Criterion aren't "hidden gems", but in my opinion they deserve to be more widely used.