have a look at
VoIPtalk although you appear to need an IP phone to plug into a router. They are slightly cheaper than Gossiptel for Ip to PSTN calls. The Gossiptel PC application can probably be re-configured to work with VoIPtalk's service, and you can use an IP phone with Gossiptel. Gossiptel sell a converter to alow an ordinary PSTN phone to be used over both the PSTN and over IP.
I found Skype for PC (Win2k) worked staright off the download - a friend also found the Mac version worked, but I was unable to get teh Linux version to work with RH(, possible because of a dependency. I haven't tried it with FC2 - I suspect it would be more successful.
As I said earlier, from a purist point of view, I prefer SIP as it is an open standard and should work across different service providers - still a little hit and miss. One snag is that if you use an IP phone with VoIP talk, as it is numeric input only, you can't enter a URL to access a subscriber on Gossiptel (to get to their server) altrhough VoIPtalk say they are entering an agreement that will allow the entering of a short numeric code will allow cross vendor phone calls. Bottom line is to look at all the sites, and make a decision based on what fits your requirement!
The box said "needs Windows 2000 or better" so I installed Fedora Core 6