I really like the Project Euler website but its emphasis seems to be more on math than programming. While solutions are given in code, the most efficient tend to rely on knowledge of different areas of math.
Is there anything similar that is oriented towards programmers? For (a dumb) example a challenge may be: "Write code to reverse a string", with people then voting on the best solution.
Ruby Quiz (http://www.rubyquiz.com/) sounds similar to what you're looking for, although it's Ruby-specific
http://acm.timus.ru/ is another website with programming problems. Almost all of them are harder than anything in Project Euler.
There is rarely a single "best" solution to a problem like this. Code golf sites will let you find the shortest-source-code solution. Many other sites (ACM, UVA, etc) score based on the efficiency of your solution for known or unknown test cases. Very few sites score for readability, maintainability, robustness, etc.
Particularly, go to TopCoder and select Competitions > Algorithm from the menu (or browse around to find the type of competition that interests you). You can go into the arena and view all of the old problems from past competitions once you sign up.