Memory Monitor is a very small, open-source system utility. It serves a single purpose and that is to show your Mac's memory usage on the Dock. Its icon is actually a real-time graph that displays how much memory is free, wired, and active. The application uses different colors in the graph so you can know your memory usage at a glance.
If you right-click on the Dock icon, you will be able to launch the Preferences window. From here, you can customize the application quite a bit. You can change the colors used for wired, active, inactive, free, pagein, and pageout memory. The refresh rate is 1 second by default, but this value can be changed using a sliding bar all the way down to 0.1 seconds or up to several seconds. The application allows you to have the memory usage graph as a separate window, and it can be configured to be always on top of other applications. The size of the window can also be customized from the Preferences window.
Version 1.2.6 works on Mac OS X 10.3 to 10.7 according to the developers. There is an older version also available on the website that works with 10.2. I don't have Mac OS X 10.8 (Mountain Lion) to test this, but it might work.
In conclusion, if you need to be always aware of your memory usage, Memory Monitor is a nice tool to have. I would rather see numbers instead of colored-graphs, though.