Android/BlackBerry/WinMo: If you're headed on a road trip with friends or moving around town in a large group, Loudtalks is a mobile app that will turn your phone into more of a walkie-talkie so you can speak to the others in your party instantly, without having to call them first.Loudtalks works with Android phones, Blackberry devices, Windows Mobile phones (note: not WP7 phones) and Windows computers. The lite version of the service is free, and allows you to create public channels for up to 100 people to join into on their computers or mobile phones. You can also map specific hardware keys on your phone to engage push-to-talk, so you can easily chime in on the conversation.
The developers behind Loudtalks also offer a paid enterprise-size service if you need to host the solution yourself or want total control over who logs in and how many channels there are. The lite version is still in beta, and while free doesn't let you control who enters a channel, or give you control over who's in one. You can, however, control whether everyone in the channel or only you can speak.