To use this on your system, start by downloading the script (226KB download). Mount the disk image, then double-click the script to open it in the Script Editor. The sender and recipient are specified near the top of the script. Change the address in quotes after theSender to your email address. Change the address in quotes after recipAddress to the address you want to send attachments to. You can also set a common name and text for the email message body. If you want to change the subject, just change the text in quotes after the word subject.
Save the file. The proper place to save it is /Library -> Scripts -> Folder Action Scripts, but you can save it anywhere.
Now create a folder; you'll probably want it on your Desktop. Control-click the folder and click Configure Folder Actions. Use the plus sign button at bottom left to designate the folder you created. Use the plus sign at bottom right to designate the script you edited. Alternately, just click Attach a Folder Action after control-clicking the folder to attach your script. If you don't have the Attach a Folder Action menu, just Control-click any Finder folder and select Enable Folder Actions.
There is also a script (10KB download) that prompts for a subject before sending the attachment. This would be useful for archiving files in your Gmail account. Drag files to the folder, enter a meaningful subject in the pop-up, and you can always search Gmail to find the file later.
[robg adds: If you have multiple email accounts, you could create multiple versions of the script, and attach each to a different folder.]

