Although you can perform system administration through the utilities supplied with Mac OS X, several third-party applications provide convenient frontends to these utilities:
Kristofer Szymanski's Cocktail (http://www.macosxcocktail.com/) is a shareware application that provides a GUI frontend to a wide range of system administrative tasks and interface configurations.
Tynsoe.org offers the free GeekTool (http://projects.tynsoe.org/en/geektool/), which can redirect the output of system logs, Unix commands, and dynamically generated images to the desktop.
Brian Hill's MacJanitor (http://personalpages.tds.net/~brian_hill/macjanitor.html) is a freeware application that does one thing and does it well: it runs the periodic jobs that are scheduled to run in the wee hours of the night by default (see "Scheduling Tasks" in Chapter 4).
Marcel Bresink's TinkerTool System (http://www.bresink.com/osx/TinkerToolSys.html) is a shareware application that can do many things, including run the periodic jobs mentioned earlier, manage log files, and tune the network configuration.