Also some of the newer Nokia phones work as a SIP client over wifi. I use the E65, which I use to connect to a uk SIP service over wireless internet, and gives me dirt cheap phone calls back to the uk and vise versa, without a PC.
The SIP services vary in price, with some you can get unmetered calls to UK landlines for about 5 or 6 quid a month, but the handiest thing is you can get a uk local number anywhere you want it, so I have a SIP number in the same exchange as my folks, which I can connect to from anywhere there's wireless internet, which helps to keep in touch.
One caution, though, you have to take care where you get the phone from. Vodafone nobbles the Nokias so that SIP doesn't work over wifi, phones from Three work fine, though.