FileMerge is great, but I often work from the command line and wanted a way to launch and supply arguments directly. (I also hate how the open file panel in FileMerge doesn't retain the last used directory for Left and Right separately and instead combines the most recently used between the two. I usually diff files from different directories so I hate having to rehunt down my location every time.)

It was a while before I learned of the existence of 'opendiff' which is the command line way of launching FileMerge. For me, it is a much faster way of specifying the targets I want to compare.