I'm surprised there hasn't been any mention of giffgaff on here. Giffgaff is a new MVNO supported by O2. It's a "community" operation. The users provide support etc. They even have the users doing the marketing. This keeps the costs down.
In return for marketing, you get a year a free unlimited calls, texts and Internet. They are doing this through a "Tool Hire" arrangement. Hire a tool, film yourself using it, upload it to YouTube and get rewarded.
This my attempt with one of their available tools: