A new OJ is available now to practice. That is www.lightoj.com and it is made by Jane Alam Jan who is shortly going to Google as well as 1 time World Finalist.
Practicing at lightoj is so good for beginners as it is categorized and have also chat room and forum which helps to communicate with others about any problem.
The another special fact is that, its itself has a C/C++ Compiler which really helpful for anyone who don't find any compiler in his PC (at campus or anywhere else). May be this will also be helpful who wants to code by mobile.
Look at once and start practicing at lightoj. :)