My Mac is dead silent. Even more so after I replaced the already quiet hard drive with a completely silent SSD.
With no audible indication of e.g. swapping/thrashing I was in dire need of some kind of system monitor, capable of at least displaying CPU and memory usage. I wanted the monitor to visible on the desktop all the time but at the same time I’m quite stingy with my screen real estate. This disqualifies CPU monitors implemented as Dashboard Widgets since Widgets are only displayed on the screen when a special key is pressed. It is possible to hack OS X to always display widgets on top of the desktop but widgets steel screen estate. I’m not a big fan of widgets anywhere really – not on cell phones, not in Vista nor in OS X. I find them distracting toys for the most part.
The option I finally settled on was Menumeters which is informative, unobtrusive, pretty (enough) and highly configurable through its own pane in “System Preferences”.
MenuMeters lives (surprise, surprise!) in the rightmost portion of my Snow Leopard menu bar thus preserving my screen estate. Wide-screen monitors generally have ample free space in the OS X menu bar.
Slackhacker 
You may consider this alternitive: http://www.islayer.com/apps/istatmenus/
Thanks for the suggestion! Istat menu looks like a worthy contender – I especially like the Istat “current time” monitor with one-click access to a calender. This one was actually some thing I missed from Windows.. đŸ™‚
PS. In the unlikely case someone is unaware about how to rearrange stuff in the OS X menu bar I’ve just found out that it’s as simple as Command-Click and drag! (Works for both third-party and built in apps)