The Gizmo Project is a granddaddy of the VOIP field. Run by SipPhone, it’s fully SIP-compatible, and useable (with an ATA) with regular telephone handsets, as well as SIP-compatible softphones.
For an indeterminate length of time, they’re offering ten free minutes of calls per day, to landline and some mobile numbers. The catch is, apparently, that it only works thru their website, and not for people with ATAs, or who use Asterisk. It’s not that bad, though; a good quality headset and sound card will produce much, much better sound quality with VOIP than traditional PSTN hardware.
It’s obvious they’re trying to attract Skype users over to the open-standard side of internet telephony, and that’s a good thing. I’ve used Gizmo for almost a year, as my primary outgoing long-distance provider, as well as for incoming calls, and have had no complaints whatsoever. They’re definately geared towards the tech-geek end of things, and most of the really cool stuff you can do with their service is only mentioned in their forums, but their rates are good and their service is excellent.
If you haven’t tried VOIP, give it a shot. You might well be impressed.