I’ve used a variety of computer based VOIP over the years. My current preference in this space is Skype as it’s free for computer-to-computer calls and even allows calls to and from regular phones (at a fee).
Video and text chat are also provided, as is ‘white-boarding’ with additional plugins.
Skype is available for most platforms, even mobile phones… I’ve found that while the Windows beta versions has some additional features, the Linux release is rock-solid.
There are also several hardware based solutions, like Vonage, available to replace your traditional phone service… perhaps we’ll cover that later.