One is TunerFree MCE, which not only streams live TV services but also gives you catch-up content from the BBC, ITV C4 and Five. If you're in the US, it supports Hulu, too. It works pretty well, although the interface is a bit clunky.
Another is the TVCatchup plugin which is at the moment a very basic and incomplete tie-in to the TVCatchup web service. It works fine for live streaming, though. According to the website, it is developing a desktop application with PVR capabilities, but there's no sign of that yet. There is an iPhone streaming service, available at http://www.tvcatchup.com/iphone.html.
If you just want free live TV on your desktop, Zattoo is a great free app, and isn't just limited to UK channels.