My friend Kyle recently turned me on to Project Euler. The site is a forum and list of problems and is a great way to get you into some math and numerics hacking examples. The problems are supposedly all solvable by a reasonably-powered computer in about a minute but there’s no proviso for you being able to code up the solution to them in that short a time :) You don’t get to see the forum for a particular problem until you’ve submitted the right answer to that problem, though, so it encourages you to work out at least a brute force solution before seeing what others did.
I’m pretty surprised at how many people chose “Assembler” as their preferred language. Me, I’m not that hardcore: I’m posting my problem solutions in Ruby as I finish each problem.