Office Communicator 2007, from Microsoft, is a communications product that unifies voice, e-mail, video, and Office Outlook. My company sells and installs it, and we've installed it in our company. I'm a usability guy who's been pretty critical of Microsoft products in the past, but Office Communicator is very good. The quality of the VoIP calls is still not as good as landline calls, or even as good as Vonage, but it's getting better. It's just a matter of time.
The Office Communicator Server also comes bundled with Microsoft Speech Server. You buy OCS, you get speech recognition for free. If you want to play with speech without paying out a bundle to Nuance, it's a good way to get started in speech recognition.
My associate Marshall Harrison blogs about OCS and Microsoft Speech Server. If you're interested in speech his blog is a good resource.