Here is a range of software (often free) which will enable to start making calls using VOIP technology and benefit from the cost saving possible.

Skype is a very popular VoIP system, with free calls to other Skype users (calls to conventional landlines are also possible at much lower rates). Ideal for beginners.

Gizmo is an open source VoIP system which is similar in functionality to Skype but has a few more features, SIP compatibility and can call landlines at similar rates.

Vonage offer a more dedicated system for unlimited local and national calls to landlines and other VoIP users. A free adapter converts your landline phone to plug directly into your router to provide a familiar service.

XTEN offer a free software which can be used on most VoIP networks

The software you choose will depend on how often you want to make VoIP calls and the type of equipment you have. If you are a heavy usage home user, then it might make sense to go with a dedicated flat free provider which allows you to use your current landline phones. However, if this will be your primary system you need to make sure you have enough bandwidth to make and receive the calls.