I’ve just discovered Project Euler (from Osfameron, via Planet Haskell). This seems like great fodder for recreational mathematics and Haskell programming. Some of the simpler questions I might even co-opt for interviews (I have been known to ask the interview question “How many trailing zeroes has 100!” which has a similar flavour).
I haven’t programmed any solutions yet, but I did just solve number 5 (“What is the smallest number divisible by each of the numbers 1 to 20?”) using Google calculator.