I've made extensive use of ideone in the recent C and C++ tutorial, as it provides an excellent free service. However it's not the only player in town, I've also mentioned codepad.org. They are all listed on the List of Online Code repositories (link below).
Thanks to the IsoCPP.org website (the home of C++ on the web), on their online C++ compilers page, there are a few more to add in which I've done.
These new ones are significant in that they include recent versions of Clang, GCC and even Microsoft VC++ 2012. Now you can test out new features.
- Link to List of Online Code repositories