Apple provides a system-wide menu called “Script Menu” (just as the sound volume menu), usually located within your /Applications/Applescript folder.
This menu will list any compiled script located within your Scripts folder, and simply choosing one of them will run it.
To install this menu, just drag it to the menu bar. To uninstall it, press the command key and drag it out of the menu bar.
After Tiger, we have now the “AppleScript Utility”, which will automate this process for you.
If you create a folder called “Applications” within your scripts folder, and create a folder called “Finder” into it, when the Finder is the frontmost application, you will see listed the regular items and the individual scripts within such “Finder” folder. This works for any application.
An alternative to this menu is FastScripts, from Red Sweater Software, which features fast running, smart switching, item revealing and running of applets, droplets and shell scripts.