Composers
Adobe GoLive, Cool MMS, InterObject MMS Composer and Player, LimSee2, Motorola Messaging Suite, Nokia Developer's Suite for MMS, Sony Ericsson Picture Messaging Home Studio
Providers
Anny Way, Comverse, Ericsson, IMImobile, Jataayu, LightSurf, LogicaCMG, Motorola, Nokia, Now SMS
Celtius Ltd
Produces a MMSC which is designed for use in development and testing of MMS related services and client devices.
Future MMS
Send Multimedia Messaging Service (MMS) messages over the internet. Custom development, easy sending via HTTP push, simple signup.
Hellkvist
Produces Mmslib which is a PHP library for encoding and decoding MMS.
NowSMS Forum
Although this is a forum for NowsSMS's product support, there are some very useful information here.
Openwave Developer Network - Tools & SDK - MMS
Describes how to access its MMSC server for testing end-to-end delivery of MMS messages.