If you can find it, you can also use the Easy Open control panel, which adds an "Open File With..." dialog to the contextural menu. You have to add the programs that you wish to open your files with under the control panel settings, but it's infinitely valuable once it's set up.

This can backfire, and you can get generic incorrect icons too. It helps to use binhex files, or stuffit archives and decompress on the Mac you wish to use. You can rebuild the desktop at startup to fix all the new icons (Command-Option held down before desktop appears)

There are also dedicated utilities for changing type and creator codes, like FileType and FileTyper, which offer more convenience than ResEdit.

Once you know document types (such as MP3 for your favourite player), you can create a droplet that does everything automatically (in bulk).