Google Chrome Auto-Close Download Bar
Easier keyboard shortcuts (Windows): Ctrl+J, Ctrl+W (open "Downloads" tab and close it, which happens to also close the download bar in the original tab).
For Mac, use ⌘ (Cmd)+Shift+J.
2019 Edit: Since it seems the extension below is no longer available, someone has mentioned this extension could work: Always Clear Downloads 2
Original 2012 Information: Found this Chrome extension today. It's relatively new, available since May 22, 2012:
Always Clear Downloads
A Google Chrome Extension to always clear the downloads list of all completed downloads.
User Reviews:
First extension that clears the download bar and hides it afterwards.
I have been looking for an extension that does this! Thank you. Works very well hiding the download bar after the download has completed. Saves a click.
For what it's worth, you can close it with the keyboard by doing:
Windows: CTRL-J, CTRL-W
Mac: CMD-SHIFT-J, CMD-W
The first command pops your downloads in a new tab, the second quickly closes the downloads, sending you back to your previous tab. Doing so closes the 'Downloads' bar.