Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Earlier, many folks were unhappy about the dozens of applications that ate up valuable space on their system tray. Often, people would have overly crowded panels that would look ugly in most cases. To address this problem, Canonical came up with Indicator Applets. It was not only a huge step forward in usability; it was also the foundation of a more clean and uniform user interface.
Unlike Windows, where you're clumsily right-clicking the tray, indicator applets allow you to interact with multiple applications at once without clicking more than twice. Now, if you take a look at the top panel, it looks much more uniform, with properly spaced icons and easy-to-navigate menus. As Canonical has released the API out in public, many developers have come up with some nifty indicator applets.
So, if you don't know which ones to try out first, here are some of the best indicator applets out there for Ubuntu: