Zingg! is a simple contextual menu for the Finder. (Version 1.4.1 is for Jaguar, Panther and Tiger (PowerPC only); version 1.1 is for Mac OS X 10.1.x.) It shows you a list of all applications that claim to open a particular file:

A similar capability is now built into Mac OS X from 10.2 (Jaguar) on up. However, Zingg! lets you configure certain applications to appear always or never in the pop-up; you can also set it to not show Classic applications when the Classic environment isn't running. There are also several other options to cut down on the size of the menu:

Zingg! is freeware. However, if you think it was useful to you, you may contribute with a donation.