conaito is offering an ActiveX SDK for VoIP applications. I've yet to try it but it can be used with VB6, VB.NET, C#, C++, Delphi, and Javascript and I've seen it for roughly $369 per license.

Description from their software:

"conaito VoIP ActiveX library for developers of VoIP audio applications, such as voice chat, conference, VoIP, providing real-time low latency multi-client audio streaming over UDP/IP networks. Includes efficient components for sound recording, playback, encoding, decoding, mixing, resampling, reading, and writing wave files, mixer volume controls access. Provides UDP/IP server and client components for peer-to-peer, multi-user, and broadcast audio communications. Really easy to use! Working with Firewall and NAT! conaito VoIP SDK is the best way to add voice conference and text conference into your application and web pages. conaito VoIP SDK includes voip activex, voip ocx, voip com, voip dll, voip lib and voip cab, so you can use the one you like the most"

Check it out on http://www.conaito.com/voip_enterprise_ueberblick.asp.