In addition to their commercial products, MainStreetGIS has several free ArcGIS extensions available. The examples are oriented around municipal applications, but you can probably find other uses for some of them as well.
MainStreetAbutters: Choose a vector layer (point, line or polygon), a buffer distance for that layer, and a target layer. The extension will select features in the target layer within buffer distance of the buffer layer, and export them to another layer and/or a table or document file like an *.doc Word format file (the latter for use in merging with other documents like mailing labels, letters, etc.).
MainStreetSpatial: Selection of features in one layer based on their spatial relationship within another layer (intersection, lying within, etc.).
MainStreetVisibility: Stores a visibility setting (layer checkbox settings) for a project. Create multiple visibility settings for single project to turn multiple layers on and off.
MainStreetParcelSearch: Load a list of parcel IDs from a Parcel shapefile layer, select one, and have the map zoom to that parcel.
MainStreetStreetSearch: Load a list of street names from a Street shapefile field, select one, and have the map zoom to that street.
MainStreetAddressSearch: Loads a list of street names and street address numbers from a Parcel shapefile layer, then zooms to a selected street address on the map.
All extensions at the above links are for ArcGIS 9.x, but versions for 8.x are available on request from MainStreetGIS.