Here is an interesting freeware app which converts your transparent Windows 7 taskbar into a dynamically changing colored RAM meter.
RAM Taskbar for Windows 7 monitors your RAM usage and converts the values and displays them in varying colors depending on the RAM usage. Green denotes Normal, followed by Orange for Warning, and then Red which stands for Critical or high RAM usage.
The app offers quite a few customization options. You can make it display the total memory, physical or only virtual memore; change color codes and so on.
Download page: DeviantArt.
If you like to keep an eye on your memory usage constantly, this app is definitely for you!
UPDATE: The RAM Taskbar is now RAM CPU Taskbar as it monitors both the RAM and CPU!
Temp Taskbar Meter may also interest you!