Menu Bar items in Mac OS X are those little icons that live in the upper-right of your screen and do useful things like show the clock, change the sound volume, switch displays, etc. A while ago I bookmarked an incredible list of menu bar items, and I think it deserves a link here, too. Personally, I use MenuMeters a lot.
It’s tough to not use a lot of these items, but keeping too many starts taking up a significant portion of your menu bar. However, applications’ menus do take priority over menu bar items, so they will simply be hidden if an application (iTunes comes to mind) needs their space.