Just a limited selection of sites, apologies if I've not mentioned yours (it isn't personal).

CreatureLabs/CyberLife
The owners of all things Creatures, and my former employer for 7 years.
Nicely Crafted Entertainment
My current employer, and home to a fair few ex-CL people :-)
Creatures Developer Network
The hub for developing things for all creatures titles.
The High Tech Hand
A great source of unique agents.
Alien Creatures
Alien provided the German translation for Docking Station, and also runs the Norn Adoption Center.
Creatures France
Pasto provided the French translation for Docking Station, and also has a good list of agents.
The Norngarden
The creator of the fantastic Norn Garden, and a bunch of other great things.
Albia 2000
The creator of Montu for Creatures 2, and a more besides.
Creatures MainFrame
Hausemouse's C3 Agents
Chani3.com
Welcome to Twilight
Random Access Creatures
Emmental's Docking Station Agents
AardLunacy

As with all things web related, these links may become dead or out of date. A good place to find agent sites is to use the CDN's classifer list to show who has created agents, and where they can be found.