Voipbuster and the other Betamax companies used to give UK incoming numbers for free (I got one). They then stopped giving them, and subsequently they started issuing them for the princely sum of two Euros.
Expect rock-bottom (or totally free) prices, good-to-great call quality and reliable operation, but virtually no customer service (though they do have a user forum). Nevertheless, it's well worth it IMHO, because it's cheap and it just works. Plus you can even use it with a slow connection, as there's the "phone to phone" option that allows you to connect any two phone numbers.