ESRI Arcscripts
ArcScripts is a collection of scripts written by Environmental Systems Research Institute, Inc. (ESRI) staff and users like you. These scripts are designed to work with ESRI software to extend functionality or automate common tasks.
Download from the source: http://arcscripts.esri.com
ET Spatial Techniques
Useful Tools, Wizards and Editing Tools, some free and downloadable, designed to extend the capabilities of ArcGIS and ArcView 3.x
Download from the source: http://www.ian-ko.com
XTools Pro
XTools Pro is one of the most popular extensions for ArcGIS Desktop. XTools Pro contains many different tools for feature conversions (e.g. convert polyline to polgon), table operations (e.g. calculate area, perimeter, length, acres, etc.), misc. general usage tools (e.g. create Personal Geodatabase, delete dataset, etc.), surface tools (e.g. convert grid to contour, build Thiessen polygons, etc.).
Download from the source: http://www.xtoolspro.com
Hawths Analysis Tools
Extension for ArcGIS (specifically ArcMap), to perform spatial analysis and functions that cannot be conveniently accomplished with out-of-the-box ArcGIS. Contains three types of tools, from simple tools that automate mundane tasks, to random point generation, to tools that target specific, ecology related analyses, e.g., various movement model applications.
Download from the source: http://www.spatialecology.com/htools