Dropbox - I use this for quick backups, transferring files between computers, and sharing files. Dropbox is perhaps the most frequently used item in my menu bar.
Meteorologist - I am a weather junkie, and my office at work has no window, so it is very handy to have a quick way of seeing what the weather is outside. Meteorologist is both simple and powerful, but I am always looking for something even better. The temperature is usually in Celsius, but when I am in the US I prefer to use the local units.
SlimBatteryMonitor - This lets me know how much power I have left. I prefer SlimBatteryMonitor to the Mac OS X default battery monitor because it takes less space, and it provides more information.
Time Machine - This lets me easily control Time Machine, and it lets me know if a backup is in progress.
AirPort - The wireless network at work is flaky, so this lets me know if I am connected, and it gives me an easy way to control my wireless connection.
Time - This gives me the time, and nothing but the time.
Fast User Switching - I have multiple accounts on my laptop, so this is very useful.
Spotlight - This is my go-to method for finding things on my computer, but it would be nice if the interface were more powerful.
(From Stephen Holland, Astrophysicist working on gamma-ray bursts, supernovae, and globular clusters)