[Jtsummers]

Not a skill tree, but this reminds me of the challenge map from http://hacker.org.

EDIT: To add, the intent of the challenge map is that many of the challenges build directly or indirectly on past challenges. Basic crypto challenges (by basic, I mean simple substitution ciphers) to simple XOR ciphers with increasing levels of difficulty (intended, sometimes there are shortcuts the designer didn't perceive). Programming challenges using their own esoteric languages or solving problems with potential exponential algorithms (until you figure out the shortcuts) and the like.

[ryalfalpha]

Definitely recommend this site, it was a big influence in my early programming days, I found it way more interesting than Project Euler.

[Jtsummers]

Agreed. I didn't get too far in Project Euler myself before finding other programming problems to solve. Logging in I only solved 4, apparently. Which doesn't seem right, maybe I accidentally created a second account, I've done that a few times.

The main issue was, though I have a math degree, there was no way it would appeal to most of my friends when we'd turn these sorts of things into a (mostly) friendly competition. Hacker.org hit just the right buttons to keep a few of them interested long enough to even plough through the problems they didn't enjoy just to make progress.