Nearly every VOIP related startup has their own click-to-call widget, Jajah, Jangl, Jaxtr, and even GrandCentral. These widgets let you easily and sometimes anonymously set up a call with friends over the web. They're very useful and come packed with features like voicemail and texting. However, each of these services connects phones to phones, which still eats away at your mobile minutes while you're talking to that business contact or MySpace hottie.
TringMe offers a bit more flexibility. Callers can ditch their phone and call directly through their Flash widget to your mobile phone, landline, and GTalk (Yahoo and Skype coming soon). All they need is a microphone and one click. Although they're still in private beta, you can try the demo widget to the right for an idea of the experience.
Similar to the other services, your phone number is kept private and the calls are free (now's the time for that overseas call). You can also set the widget to just receive voicemails, which are emailed to you, saved on your standard mailbox, or recorded and played back in GTalk. There is one major drawback, though. Since there is no virtual phone number involved, callers have to be at a computer and can't call you while they're on the go.
Naturally such an easy and anonymous calling service is susceptible to abuse, and I don't see any countermeasures in place to keep out prank calls and telemarketers. The other services have verified phone numbers and white/black lists to keep abuse to a minimum. I expect TringMe will have to incorporate similar controls to make people more comfortable with using the widget.