You've probably heard of these sites over and over again, but I'll give you the benefit of the doubt and list my favorite Creatures sites for you anyway.
This community-constructed site contains all the information you could ever need for all the Creatures games.
A cleanly designed community website, my choice for forums and news.
An open source, cross-platform version of the Creatures Evolution Engine. It's not quite mature enough for normal use, but I'm keeping an eye on it. This project looks to be the best thing to happen for Creatures since sliced... cheese.
For budding developers, this is the prime location for information and tutorials.
Agent-making tools written in Java, for you cross-platform development needs. So far it includes a sprite editor and a PRAY compiler/decompiler. Decompiling is actually an incredibly useful thing for learning -- you can look at the source of your favorite agents and see how they work.
Classifiers uniquely identify each agent in C3/DS. If you plan on distributing your creations, make sure to reserve some script numbers first.
Blender is an open source, cross-platform 3D graphics program. 3D modeling is a skill like any other, and Blender does take a little effort to get the hang of, but with perseverance you'll be making great looking agents in no time!