gAttach!  allows you to easily attach files to new messages in Gmail or Google Apps Mail.

All the features available in Windows that were once only available to your desktop mail-using friends are now available to all Gmail users.

Website: http://www.gattach.net/index.html

Since moving to Gmail for all of my email accounts I have been on the lookout for various add-ins and tools that would enhance my email experience. One of the best tools I have found is Blank Canvas Gmail Signatures. 

Blank Canvas Gmail Signatures is a Firefox extension that automatically inserts HTML signatures into your Gmail messages based on which address you are sending from. Works for Compose Message and Reply/Forward.

From the Blank Canvas Gmail Signatures home page this plug-in provides the following features:

• Works for Compose Mail, Reply, and Forward
• Support for single or multiple email addresses
• Create a different signature for each email address you use in your Gmail account
• Supports up to four signatures per address
• Real-time signature preview while editing signature HTML
• Signature automatically inserted into message above (or optionally below) quoted text
• Support for special characters
• Easy setup and configuration interface