But it certainly is the case that, at times, Safari starts to take much more of my precious RAM than I want it to. I don't mind too much if it is taking 100MB if I've been looking at graphically-rich pages, but I expect it to be a lot less than that most of the time. And if I notice that Safari is taking up much more than 100MB of real memory (e.g. by looking at Activity Monitor), I will quit and re-launch it in order to conserve RAM.
So I wrote an AppleScript that monitors the RAM usage of Safari (or other apps) and displays it (in megabytes) in the title of the frontmost window. The AppleScript pops up an alert dialog if the Safari memory usage exceeds a specified amount (currently 100MB). There is also an option to use Growl for these warnings. If Safari's CPU usage exceeds 10%, the CPU usage is also shown in the title bar.
To use this AppleScript, you need to copy and paste it into Script Editor, and then save it as an application bundle, being sure to click on the Stay Open checkbox. After that, you run it like any application -- e.g. by double-clicking its icon. The initial version of this script was posted to this thread on the MacOSXHints forums, and has benefitted greatly from suggestions made there by 'NovaScotian,' 'bramley', and 'mark hunte.'
[robg adds: To get this to compile, I had to download Growl, so that Script Editor would find the Growl helper app. After that, it worked as described, though in Mail, the RAM usage seemed to come and go somewhat randomly.]

