Laurie covered MenuMeters as part of Freeware February back in . . . er, February, but the other day, someone in the comments to one of my posts asked what the little graph and arrow thingy was in my menu bar, and I neglected to answer.
Fortuitously for that TUAW-reader and commenter, the recent update yesterday of MenuMeters reminded me of the comment and gave me the opportunity to post about it here. MenuMeters version 1.2.1 is freely available under a GNU General Public License, and is a very useful tool for monitoring your system's behavior. Think Activity Monitor meets menu bar.
MenuMeters can display a CPU Meter, Memory Meter, Disk Activity Meter, and Net Meter in your menu bar with more detailed information via drop down menu. You can customize which meters you want displayed via the MenuMeters Preference Pane which is installed in your System Preferences. I've included a picture of the Memory Meter along with this post.