CounterPath have produced a few programs for use with a Laptop or PC.

X-Lite the free version of the VoIP SIP software and EyeBeam costing only £17.94 for 1 License.

These are ideal for people on the go that use there laptop a lot out of the office or as a starter solution for a new VoIP account to test the service.
It is ideally used with a 3.5mm jack headset that just plugs straight into your laptop or PC.