FWD is OK for free calls (you can call US, UK, Dutch, and Japanese freephone numbers) but it doesn't give you the option of making other PSTN (Public Switched Telephone Network) calls.
Once you sign up with FWD, you can get a free US number, by signing up with
ipkall. You'll have a number in Washington State +1 360 227 xxxx
Services like these are only just taking off in the UK.
Gossiptel lets you receive numbers on an 0870 number (yuck) but will shortly be offering geographic numbers as well. That means if you live outside the UK you can receive calls on it for free (with 0870), or for a small monthly fee (geographic).
Im looking to be a reseller for a company like this, and provide an analogue telephone adapter, which means you don't need to have your computer on to make calls. The list of alternative numbers to 0870 ones is ideal, as I can program adapters to translate any 0870 or 0845 number back into the geographic one!