There are several VoIP solutions that you can try, so I’m going to subdivide these into some categories for which is best for which situation:

PC to PC
Skype is easily the all out winner. Availiable for damn near everything, great quality, near perfect connection and call completion rate. I’ve rattled on about this before. PC to PC calls, Skype it is.

PC to Phone
VoipBuster

Download it (I have, its a clean, simple program), install it, create an account. You can then make 1 minute free test calls. If you put EUR1 into you account, you can make unlimited length free calls to a lot of countries, including Canada, USA and UK!

100% Free, no call setup!

This was recomended by the good people on the Skype Forums, the Xten Support Forums and VoIPUser.org. VoIPUser has a guide to getting X-Ten’s X-Lite and X-Pro working with it.

Phone to PC
For the UK, this would be SIP Gate they will give you a free phone number in nearly any area code, and when configured to work with your favorite SIP client (or hardware), such as one of X-Ten’s clients, you can take incomming calls. X-Lite and X-Pro will let you set up multiple SIP accounts in them, so you could use VoIP Buster for outgoing and SIP Gate for incomming.

Enjoy, I’ll be using a fair few of these in Canada