XviD is an open source MPEG-4 video codec, written in C with assembler optmizations for quality and speed (including MMX, SSE, SSE2 and 3Dnow! code for i386 and AltiVec for PowerPC). [edit]
If you find yourself increasingly working with video online, you're not alone. It's gotten much easier to add video functionality to applications, blogs, and much more. The open source community has created many excellent applications and utilities for working with video and animation. Over on WebWorkerDaily, I have a post up titled 6 Free Apps and Utilities for Working with Online Video. It covers top-notch, free applications you can look into, including Blender, Xvid, and more. Not all of them are open source, but most of them are. Check it out.
Whether you do some blogging, operate an e-commerce site, or simply have moving pictures in the back of your mind to work with, web-based video and animation should probably be on your radar.