What's a good open source VoiceXML implementation?

Voiceglue (http://www.voiceglue.org/) is an implementation of voicexml using openvxi and asterisk. It may be a good option for you, it is GPL licensed.

You can check out Asterix - http://www.asterisk.org/ for an open source solution.

If you want to build an IVR and you're not married to VoiceXML, you might try Twilio. They have a simple XML syntax, an awesome REST API, and small-project-friendly pay per minute pricing which lets you concentrate on building your app and not building/hosting telephony infrastructure. I built an IVR app using their system a few days and it was a pleasure.