RFind is a little application that indexes the filenames of a given directory, and allows to quickly search this index with regular expressions.
The motivation behind this app was that someone thought this had to be in C++ to be fast, so I proved him wrong: search-on-typing with more than 500,000 indexed filenames is easily possible 
I have tried to make this little tool very configurable so that is can be useful to everyone. Some of the features are:
- Hierarchical presented search results
- Search-on-typing
- Define rules to execute on mouseclick
Download:
This tool is written in Ruby and requires fxruby 1.0, which is a bit out of date. I will try to update it to a recent version of fxruby soon.