Future MMS: Send Multimedia Messaging Service (MMS) messages over the internet. Custom development, easy sending via HTTP push, simple signup.
GSM World on MMS: Articles on MMS.
Hellkvist: Produces Mmslib which is a PHP library for encoding and decoding MMS.
JVending: J2ME MMS Client for Mobile Phone.
NowSMS Forum: Although this is a forum for NowsSMS's product support, there are some very useful information here.
Openwave Developer Network - Tools & SDK - MMSC: Describes how to access its MMSC server for testing end-to-end delivery of MMS messages.