Automatically exported from code.google.com/p/simbl
SIMBL is the SIMple Bundle Loader.
It enables various plugins like PithHelmet to run in Cocoa applications such as Safari.
If you think you've hit a bug, check the FAQ. If you're question hasn't been answered there, peruse the Issues List.
You can follow NewDevelopments for semi-interesting information about SIMBL. There are some enhancements scheduled for the near future. The brave can try and make sense of the change log.
There are a number of SIMBL plugins to enhance a number of applications.
For more information about developing plugins that use SIMBL, you can try following the tutorial. You could also skip to the troubleshooting section.
If you are interested in modifying a particular Cocoa application, the Armchair Guide to Cocoa Reverse Engineering.