I use Zwift and it's been great. After a few solo rides I joined up on the Pacific Wake Up ride and find it is a lot more fun to ride with a group. There are also various workout modes available too.
Here's the calendar for Zwift group rides/races:
www.zwiftcalendar.com
And check out the unofficial users manual here:
Zwift User Manual - An Updated Guide - TitaniumGeek