1. Jing is a great app that allows you to capture screenshots and screencasts to share with other people. It is integrated with screencast.com to share the screenshots with others. This is great for little tutorials- if everyone asks you for tech help, you can tell them to watch a video and save yourself the time.
3. Weather Bug Lite is a great app to tell you the weather in your area. The display icon shows the current temperature.
4. SMC Fan Control is a good app for geeks. You can use it to control your fan speed, i.e. if your computer is getting warm in your lap, you could speed up the fan a little.
5. Caffeine is a great app to prevent your display from going to sleep on you. You can set it to one of the preset durations or indefinitely.
Dropbox and CloudApp (online file storage services) also have menubar icons that are quite useful. For more menubar apps, check out http://menu.jeweledplatypus.org/. If I forgot your favorite, please put it in the comments section.