Any kind of chat based or IM based system can work well for playing online. The one Ive used probably the most over the years is YIM but you have to have some way to bring the group together and make sure they can be there at the right times
If your looking specifically for new world of darkness games, click the link to Nightfall salem in my signature. Its a great community that does most of its roleplaying via chats
Also for any game made by white wolf (scion, NWoD, Exalted, etc) they do have an area set up wich lists various roleplaying sites and explains what kind of bases are used (chat, forums, IMs, something else). If you play pathfinder theres also a place on Paizo's website where you can put together group games in person or online.