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.

MMS FAQ
Offers answers to commonly asked questions.

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.