Jangl
Jangl, like Jajah, connects a call by ringing both end-points. Unlike Jajah, though, Jangl doesn't depend on you knowing the number you want to call. Instead, you enter your party's e-mail address. Jangl assigns a temporary phone number, which you call from your registered phone number. If the e-mail address belongs to another registered Jangl user, you'll be connected. If it doesn't, you'll leave a voicemail asking for a callback to another temporary number. When the person calls, Jangl will connect you.
This way people don't need to know your personal phone number. Jangl's temporary numbers are semi-permanent -- your callers can put them into their contacts list and use them over and over, but if you decide you don't want to hear from them any more, you can block the number or delete the contact entirely.
For now the Jangl beta is free, even for international calls ( currently limited to 32 countries). Jangl's temporary callback numbers are all local as well, so that won't cost you anything. So, just as with Jajah, as long as you and the person you call are both registered Jangl users, the service costs nothing.
Jangl is a little bit more cumbersome than other services because reconciling e-mail addresses adds a step or two to the process, but it's painless, and it works. You can keep your phone number private, and -- at least as long as Jangl is free -- you're not leaving yourself open to anything beyond your normal phone charges.