Various Greasemonkey scripts that enhance Gmail:

For the new version of Gmail

• Macros: adds more keyboard shortcuts (navigating to and applying labels, archiving, etc.) more info

For the old version of Gmail

• Conversation Preview: shows a preview bubble with a conversation contents while viewing the list of messages (i.e. without requiring navigation to another page) - more info
• Fixed Font Toggle: adds a font toggle button so that messages can be viewed in a fixed-width font more info
• Label Colors: allows labels to be colored so they stand out more more info
• Macros: adds more keyboard shortcuts (for marking messages as read, moving them to the trash, applying labels, etc.) more info even more info
• Google Reader Integration: adds a 'Feeds' links in the navigation area that opens up an embedded instance of Google Reader more info
• Saved Searches: lets you save search queries so you can re-run them easily more info