skype
I encourage everyone who reads this to try Skype out.
As someone with only a cell phone, I see it having a lot of potential. I month or two ago I made a crisp, clear phone call to an acquaintance in Australia in the middle of a weekday. I made the call with my computer, and it didn't cost me anything, because we were both using Skype.
Skype is not the end-all solution to Internet telephony. It is not a package like Vonage and others provide, supplanting normal telephones. It does have exceptional rates if you want to call people using regular phones, but I haven't tried this yet (I am a cheap college student). It is free and available for Windows, Mac, and Linux, so I see no reason why it can't grow to the popularity of AIM and the like.
If you would like to add me, on Skype I am michael.greene. I am always connected, though not always available. If I recognize you though, I'll attempt to return your call.
As always, cross-posted to my blog