Programmable Voice – tools that allow you to manipulate voice 

An Open Platform for Telephony Innovation. Ribbit is an open platform for telephony innovation created to change the way we all think about communications. A company dedicated to advancing communications by empowering developers to rethink what's possible by giving them an open API to build the future we've all been waiting for.http://www.ribbit.com/

Check out the chalkboard phone.

See: The talkification of the web.

Let the web stay connected to you. TringMe opens up its platform APIs for developers to empower them to create rich web-based, standalone or mobile voice applications. For example, Social networking, Dating sites can easily integrate voice as a means of communication in innovative ways.  This easy to use Adobe-Flash based web-phone can be integrated into your online world. http://www.tringme.com/developer.php

Plug-in, Plug-out or Mashup with Skype – from concept to cash. The Skype API provides a mechanism for 3rd party scripts and applications to control Skype UI functions and implement additional or improved features to complement the Skype's popularity is being fueled by an active ecosystem that includes 15,000 developers. Today, there are more than 190 Skype Extras in the client library with 25 applications certified. In the Gallery, there are over 250 software plugins for Windows, Mac and Linux users.  https://developer.skype.com/

Your voice your space.  MyVox applies voice to the burgeoning social networking and widget spaces, letting social networkers speak their minds for the first time, right through the phone. By turning the phone into a mic that anybody can use, MyVox is unleashing a new class of voice-based applications for the Web. Since its release in early 2007, MyVox has established more than 500,000 registered users. http://www2.myvox.com/

The Ifbyphone HTML request-style API provides developers with a telephony interface for custom applications. Known as Ifbyphone Glue, the combination of our API with your code allows interaction with Ifbyphone's building blocks, giving you the freedom to build a wide range of Interactive Voice Response (IVR) applications. http://public.ifbyphone.com/services/developer-api  

VoiceObjects is the cool technology in the VoiceXML world as the world’s first phone application server. http://developers.voiceobjects.com/downloads/

See demo

Rapid Development of Voice Applications.  The Nuance Café is a VoiceXML development environment for the rapid development of voice apps. As a free, Web-based development environment, the Café features a wealth of valuable tools, documentation and other resources, all with the objective of helping developers to build the highest quality voice-enabled applications in the shortest period of time. Once built, applications can be easily deployed to production and hosted on Nuance's proven, carrier-grade VoiceXML Hosting Network.  http://cafe.bevocal.com/

Open Source Telephony. Asterisk is the world's leading open source PBXi, telephony engine, and telephony applications toolkit. Offering flexibility unheard of in the world of proprietary communications, Asterisk empowers developers and integrators to create advanced communication solutions...for free. Many many web based applications are built upon asterisk servers utilizing its incredible feature set.  http://www.asterisk.org/

Not only is asterisk a fully featured BX, people have put it to use doing a varierty of innovative things:

Create a voice enabled jukebox of all your music

Add a weather forecaster to your phone.

Add voice to your plants allowing them to make a call when they need watering.  Call  212.202.8348  to hear about the plants.

Play voice enabled games over your  phone.

Automatically block telemarketers.

Open Source IP PBX.  FreeSWITCH is an open source telephony platform designed to facilitate the creation of voice and chat driven products scaling from a soft-phone up to a soft-switch.  It can be used as a simple switching engine, a PBX, a media gateway or a media server to host IVR applications using simple scripts or XML to control the callflow. http://www.freeswitch.org/

Comparing Asterisk with FreeSwitch

PingTel’s SIPxchange is an open-source IP-PBX platform that is based on Linux with integrated voice mail, ACD, multiple auto attendents, and web-based system configuration and management tool that can support 5 to 50,000 users..  http://www.pingtel.com/

Open Source Voice Applications. RocketSource is a meta-project created to bring together a community with common interest in developing open source telephony applications using VoiceXML, CCXML, and related standards. http://rocketsource.org/

The Open Mobile platform.  Enables companies to develop, deliver content, and use mobile payments with a single web centric API that connects with 60 mobile carriers.  It also enables some inbound SIP termination. http://www.zong.com

Build voice applications for any phone.  Voxeo provides a free development platform, resources, and technical support for Interactive Voice Response (IVR) and voice recognition applications, integrated with modern XML, web development solutions, and VoIP services. http://evolution.voxeo.com/

Numbr offers solutions for businesses looking to integrate telephony directly into their business applications. Salesforce.com, for instance, can easily integrate outbound calling, user-initiated conference calling, and many of Numbr’s services, simply by leveraging Numbr’s scalable infrastructure via teleAPI™, a standards-based Applications Programming Interface (API).  http://www.numbr.com

The Utterz API is now available for developers to build new applications & widgets on the Utterz platform.  Utters can be audio, video, pictures and text.  The API enables group MMS features for social web developers.   http://www.utterz.com/api/