I use Skype for video games that don't have mic support (or as we geeks know it, VoIP.) It has advantages in that if you're calling another Skype user it's free, you can conferece a bunch of people in, you can use video, and it's typically much clearer than a regular phone because it's digital, removing lots of static, and it doesn't lop off the upper and lower frequency bands phone lines use for DSL.
It's pretty cheap for actual phone calls too. You can pay like $30 a year and have unlimited calls to anywhere in the US or Canada at no additional charge. They also have decent per minute rates if you prefer to go that way, by buying "Skype Credit."
But (there's always a but) they have a pretty poor record of customer service. Lots of people complaining about having paid for the service and then getting charged for calls, or not being able to call because they've somehow used up their Skype credit even though they haven't. Meanwhile, they can't get a hold of anyone at the company to straighten things out with. And the program is ill-tempered. Sometimes it will just drop your call without explanation or warning. It occasionally will not show you as being "online", thereby making it impossible to call anyone and requiring several restarts to get it working again.
I wouldn't really recommend using it for business stuff, but if you're going to use it for largely unimportant things then you can have lots of fun with it. It's nice to be able to call other people who have Skype for free and more or less get free conference calls that way. Just make sure to have a decent microphone headset or it becomes harder to hear/be heard than talking on a cell phone in a tunnel.