First a disclaimer: We develop applications for Mac and we have done a few for the menu bar that I use daily, so I will include them.
Gnotifier: A Gmail and Google account notification system for multiple accounts (mail, calendar and documents). I’ve set it to check my personal Gmail account and two company accounts that are hosted on Google Apps. We didn’t find a solution for that, so we developed it :).
The 3 next icons are CPU LED. (Using the well known red, yellow and green colors, all the leds indicate the status of some important part of your Mac). First, Disk Usage, then CPU Usage, and lastly Memory Usage.
The next one is Adium. Having to answer chats from multiple networks, that was an easy choice.
Next, my second app to be installed on my Mac: Dropbox. Full Sync Backup Cloud Storage. No more words.
And then the last icon and the first app that I use every single day. We created it after suffering tons of lost time from missing some important piece of info: Clipboard History. A clipboard history manager with tons of features and loved by thousand of happy users.
(From Ariel Di Stefano)