I use astratel, sipphone, oztell and a few others.
While in the UAE I setup a nice system whereby people could ring my Sydney/Brisbane/Canberra number and my laptop would ring so for the cost of a local call people could keep in touch with me. I also do all my calling home from the laptop costs me 10 cents for as long as I like to anywhere in Australia from anywhere in the world as long as I have a net connection, can't beat that for value.
At home I have one Sipura SPA842 handset, a Sipura SPA3000 gateway ATA and an asterisk box, now i'm hunting cheap wildcat cards to setup a 10 line pabx and do some demos in class.
i also setup up external VOIP gateways from countries I have friends and relatives in so now from around 10 different countries people can call my number local (in their country) which is a local call for them which gets routed to my systems. So I like VOIP
One place I helped put onto VOIP for just STD fax calls their costs went from $400 a month down to $40. I know some of my family who I have setup cal overseas a lot drop from $300 a month down to $10 and they make same amount of international calls to normal phones.
Many people use Skype, it's not bad for newbies and if other person uses skype too it's free i guess but I like my normal SIP providers that I can offload onto a cheap gateway to make the final mile switch to places with no net. Sometiems you get quality problems sure especially if you're using the net at the same time but there are QOS systems you can setup to combat this.
Anyway bored at work today I think i'll go setup one of these class computers to run asterisk and teach my diploma students some VOIP configurations with PBX functionality.