BrightnessTray is an open-source tool which enables adjusting the brightness of your monitor from the system tray, and throws in some small bonus options as well.
The program arrives as a tiny stand-alone executable, barely 100KB. Run it and an icon appears in your system tray, displaying the numeric value of your current brightness setting.
Click the icon, a brightness slider appears, and you're able to adjust this by dragging it up and down. This didn't go to the full limits of our display settings - the lowest brightness wasn't black, the highest wasn't at whiteout levels - but it was probably enough for our operating needs.
Right-clicking the icon displays a "Caffeine" setting. This tries to prevent the display being turned off when your system is idle.
Other features include a "Turn off display" feature which didn't work for us, and an "Enter sleep mode" which sets your computer to Sleep mode. Why would you do that from BrightnessTray, rather than use the standard Windows controls? We don't know, either.