Dozing Cat Software has released a handy utility for monitoring a variety of aspects on OS X systems. Overseer provides the standard process viewer, but also shows system load, log entries, connected users, and more. According to Dozing Cat Software (courtesy of VersionTracker):
Overseer displays and continuously updates a variety of information about your Mac OS X system, such as processes (including CPU and memory usage), system load, users connected via telnet or ssh, free and used space on all partitions, virtual memory statistics, and system log entries. It is a Cocoa application written in Objective C and Perl and is released under the open source BSD license. See the Dozing Cat Web site to download the source code or view the documentation.
Overseer is available for free, and you can find more information at the Dozing Cat Software Web site.