SPOJ (Sphere Online Judge)
Polish SPOJ
SPOJ Brasil
SPOJ - Shortening codes (skracanie kodu)
Timus Online Judge
Themis (Polish)
UVa Online Jugde
Saratov State University :: Online Contester
Main.edu.pl (Polish)
Z-Training
Caribbean Online Judge
Project Euler
Code Golf
SIO (Polish)
Open CS
Nick's Mathematical Puzzles