How often do you find yourself wanting to check the source code of a locally installed Ruby gem? Do you find it painful digging around your filesystem to locate the gems directory?
Rather than this:
mate /Library/Ruby/Gems/1.8/gems/activerecord-2.0.2
I now type this:
mategem activerec[tab]
Note the [tab] key, which converts "activerecord" into "activerecord-2.0.2".