One of the first of these to hit the market was the Siemens Gigaset M34 USB adaptor, which lets certain Siemens phones work with Skype.

ENGADGET: Siemens' M34 USB adapter for Skype reviewed

Digital Lifestyles tried one out, and came away impressed, both by the Siemens S440 handset they tried out (which, of course, still works without Skype), and by the M34, which, in addition to Skype, allows you to IM with your phone, and even control your PC to some extent (so that, for example, you can use your phone as an iTunes remote).

ENGADGET: Siemens' M34 USB adapter for Skype reviewed