You can define a keyboard shortcut for any bookmark in Safari (in Mac OS X). Here is how:
* Go to Keyboard & Mouse preferences and the Keyboard Shortcuts tab.
* Click on "Application Keyboard Shortcuts"
* Click the + button
* Select Safari.app for Applicaton
* Put the exact title of the bookmark in the Menu Title (spaces are fine)
* Enter in your keyboard shortcut in Keyboard Shortcut
I used this to bind the Google Bookmarks bookmarklet to Command-D which overrides Safari's default (add bookmark). This is not quite as nice as the GMarks extension, but close enough.
I tried this in Firefox 3 beta 4 and I couldn't get it to work (GMarks isn't fully functional in that version currently and doesn't show up in the add bookmarks dialog).
Update: This actually does not work consistently. Sometimes it will bring up Safari's own bookmark dialog instead, in which case you have to expose the bookmarklet by going to the bookmarks menu and allow it to display the portion of the menu that has your bookmarklet.