jekyll_ext allows you to extend the Jekyll static blog generator without forking and modifying its codebase.
With this code, not only do your extensions live in your blog directory, but they can also be shared and reutilized.
More information can be found here: Jekyll Extensions -= Pain
gem install jekyll_ext
Now you just need to create the directory extensions in your blog where all your extensions will live. Any .rb file in that directory or subdirectory will automatically be loaded by jekyllext.
IMPORTANT: Instead of using the jekyll command, you should now use ejekyll instead (which is just a jekyll wrapper that loads your extensions).
To get you started, take a look at my own extensions directory.