I've implemented Cocoa services support for Mac OS X. If you drop your Factor.app into an Applications directory, then next time you log in, there will be a Factor submenu in the Services menu in each application.
Screenshots demonstrate this feature best.
So we launch TextEdit, and type a short Factor expression...
Then we invoke the "Evaluate Selection" service...
And the result is pasted right back into the TextEdit window:
One problem is that if Factor is not running, invoking the service does not launch it. I'm not sure how to fix this; I guess LaunchServices cannot locate the Factor.app for some reason.
I also implemented support for open events, so dropping a file from the Finder onto the Factor dock icon will open a listener window and run the file.