If you don't have an fmnu template (see below) and you want to change or add a command key to a menu item, you have to poke around until you find the text of the menu item's name (e.g. "Empty Trash") and then change the byte that is three bytes before the first letter of the item's name to the command key. Maybe an example would help - to use Command-T to "Empty Trash", open fmnu #1255 (ResEdit opens fmnu resources as generic resources unless you have an fmnu template, see below), and change byte number 2C to "T" or $54. Byte number 2C is three bytes before the "E" in "Empty Trash" at byte number 2F. Like most things with ResEdit, this isn't for the faint of heart, and for all I know it may have some disastrous consequences. But it works for me, and I haven't noticed any side effects.
Adding Links in Snow Leopard's Mail
Apple Mail in Snow Leopard now has a Command-key shortcut for adding a link to an email.
If you use plain-text email, this will not be helpful at all, but if you send styled email, it's a nice shortcut for adding URLs to your email messages. Simply select the word(s) you want to make into a link, press Command-K, and enter the URL to build into the link.