[mFixman]

It's a pity that nobody was able to reproduce the challenge phases of TopCoder SRMs.

Coding problems while taking of edge-cases for later was really fun.

[sltkr]

FYI, CodeForces allows challenges too.

They even implemented it slightly better: instead of having a separate challenge phase, you can “lock” your solution for a particular problem at any time (which means you cannot resubmit it) and then view other people's solutions, and challenge them with custom input.