This software does just what you think - shows blinking LED's for your hard drives with any detected activity. It has a few extras, the one I like in particular is its ability to flash the Scroll Lock LED when there is activity.
Website: http://www.myportablesoftware.com/freehddled.aspx
Download: http://www.myportablesoftware.com/free_hdd_led.zip
This software classifies itself as donationware but does say the software can be used (forever) free of charge. It does put an ugly 'Donate' button on the LED window, but it can be minimized to a tray icon.
I've tested it and can verify it writes the settings to a portable INI file. I did not bother checking whether it writes anywhere else.
The reason I was looking for something like this is that my current case doesn't have an HDD LED so I needed some indicator of what was going on. I personally think the program uses too much memory, so I may just code something using performance counters..
Oh, and there's a few other portable software programs at the site:
- Free TvDB
Free MovieDB
Free HDD LED
Free USB Guard
Free File Wiper
Free File Camouflage
Free Clipboard Manager
Free Message Encrypter
My CPU Monitor
My Memory Monitor
My HDD Speed
My Network Speed
My Startup Delayer
My Screen Capture
My Watermark
My Daily Wallpaper
My Daily Space Wallpaper
Shuffle Music Player
Fast Shutdown
Fast User Switch