Nkrs
Trying to hunt down a bug with MATLAB using wrong indices to access matrix elements, so I rewrote the entire program line-by-line from MATLAB to Go. The program gained 100 lines due to newly defined data structures, parsing CSV into them, and error checking. Not only am I getting the *correct* results now, but the program is much faster (reduction from over 5 seconds down to under 0.1 second).
Nkrs
I've been using it for long time now, perhaps I've even mentioned it in IRC. It's my favorite language to work with exactly because its standard library is enough for most applications.