Plug-in for Lightroom 2, allows for filtering by focal length metadata in Lightroom's Library Filter.
Please note that this page is a quick translation of the plug-in's main (german) page and does not necessarily reflect the latest state. new The german page now contains an extensive "FAQ and Troubleshooting" section.
Leave a message regarding software issues, ideas etc.
Work-around release for Lightroom 2.2: If you use Lightroom 2.2, please get the latest release of this plugin and then trigger a scan on your complete catalog. Lightroom 2.2's library filter does not properly match metadata strings generated by earlier releases of Focal Length Finder. (2008-12-18)
The plug-in scans your catalog and writes information about focal length into two custom fields of every single photo. Different from the existing focal length metadata, these fields can then be accessed in Lightroom's "Library Filter".
As a bonus, it collects statistical data about focal length use which can be written to a text file.
Note: Although these screenshots aren't translated yet, the actual plug-in is.
Main dialog (Accessible via: Library > Plug-in Extras > Scan for focal length data):

Scan run (as of "Preview 1.3", this is actually done in the background):

Library Filter filtering (left: using result set, right: individual selection):

Plug-in specific metadata attached to photo:

To install, add the plug-in's folder in Lightroom's Plug-in Manager.
Start scan: Library > Plug-in Extras > Scan for focal length data
Search for photos in Library Filter (see screenshot)
A focal length range may be provided to create a specific predefined result set (as well).
Searches on either all of your photos or a selection. If there is no selection, the contents of the film strip are used. (Main dialog indicates the amount of photos that will be scanned.) Please note that if you scan a subset, the "focal length matched" field will be valid for the same subset only, and all other photos' "matched" values are rendered invalid.
The count of each occurring focal length is calculated. The result can be stored in a "CSV" text file. (Format: focal length -> count, one line per focal length)

(Visualization done with OpenOffice Calc, click to enlarge)
Plug-in is localized (English, German).
Adobe Lightroom 2.
new FocalLengthFinder.lrdevplugin_Preview1.4: zip, 2008-12-21
Software has been written for my own personal use. Untested software, use at own risk.