Here's a lsit. Pick what you like:
Python: Easy to use and learn, lots of modules and documentation, very cool to program with, moderate speed
Ruby: same as Python except the modules and documentation are a bit harder to find
Scheme/Lisp: functionnal languages, lots of parenthesis, gives you a great programming experience
Sather: a language I discovered tonight: it is fast (faster than C++), safe, statically typed, OO and a not so bad syntax. THe documentation is very hard to find and very few people use it
Java: big language these days, but the support for Linux is a bit lacking.
C/C++: Very popular languages, but these are used less and less for user applications due to their natural unsafeness
Perl: Errr.... don't pick that one, unless you want to write REALLY messy code
O'Caml: functionnal language, that's very fast, statically typed.
Pike: a scripting language that's really fast, it's OO, it has good string manipulation and it has a C-like syntax, so you aren't lost if you know a bit of C/C++/Java
Emacs Lisp: learn this only if you want to implement something in Emacs. It can give you a nice intro to lisp though since it's much easier than other Lisp's.
That's it. Good luck.