Cool ArcView Extensions

GeoCommunity Portal
Tons of different tools.

ET Geowizard
Assortment of handy tools

Hawth's Tools
A must-have set of utilities for Arc 9.x

Animal Movement Extension
Does a ton of basic analyses on measuring habitat use and movement dynamics.

Under the Hood
This is a great tool to organize and access scripts and AV extensions. Read the read.me file for instructions. 

Patch and Habitat Analyst
Tools to do landscape ecology patch metrics and habitat modeling

Spatial Tools
Spatial Analyst Tools including grid clips, merges, filters, and much more.

Xtools
Erase, Identity, Intersect, Clip, Buffer, Append, and Builds from within ArcView.

Stanford University Libraries
Nice collection of links to ArcGIS extensions.

ESRI Scripts and Extensions
Massive source of tools for ArcView