Most of my menubar icons are used as indicators for monitoring certain aspects of my system. I rarely click on any of them as I do almost everything via the keyboard.
From right to left:
Spotlight: I really should get rid of that one as I never use it. Instead I’m using LaunchBar for searching.
Clock: With weekday display.
iStat Menus Calendar: To keep an eye on the date. Also helps me keep track of several world clocks.
Input Method: I’m mostly using my Austrian (German) keyboard. Also gives me access to Kotoeri (Japanese) as well as keyboard viewer and character palette.
Battery: Obviously I’m on a MacBook Pro, so having charge status visible is vital.
Volume: To keep an eye on the audio output. I use the keyboard to actually change volume. Rarely used to check audio input and output (by holding ⌥ while clicking it).
AirPort: Mostly used for monitoring my WiFi connection. Rarely used to actually connect to a network as my MBP usually knows the networks I connect to. Even more rarely used to gain technical details about the associated to access point.
Displays: Put quite on the right hand side on purpose if a projector uses a very low resolution and my Mac decides to mirror displays. That way I can still easily get to it, just in case. Think of it as an emergency icon. (I could probably get rid of that one as I usually use LaunchBar to open the Displays PrefPanel anyway.)
Time Machine: I never fully trust Time Machine. This helps me to check if it is working properly.
iStat Menus CPU Bars: To keep an eye on my CPU’s load. Rarely used to spot a CPU hogging process. (I usually do that in a Terminal via Visor. (See below!))
MenuMeters: A paging indicator (to know of excessive swapping action) and my memory usage pie chart. (iStat Menus doesn’t have a paging indicator). Too bad I cannot turn off the chart and have only the paging indicator there, or have iStat Menus give me a paging indicator instead.
iStat Menus Memory: Memory usage graph to easily spot applications that suddenly grab a huge chunk of RAM.
VPN: To connect to my customer’s networks all around and to protect my connections when using untrusted networks. (So pretty much any network except for my own.)
Dial up: To connect one of those pesky USB UMTS/3G modems.
iStat Menus disk monitors: To keep an eye on disk usage. The leftmost is my system partition and usually way too full for my SSD to be comfortable.
iStat Menus disk throughput: To check for SSD/HD/USB stick speeds during lenghty copy operations or when recovering data from faulty media.
MenuMeters network graph: Gives more comprehensive info of my interfaces and a cubic root scaled graph which I prefer over iStat Menu’s way to display this data. Also shows connection status and IPv6 info.
iStat Menus network monitor: Doesn’t really work on my MBP, maybe I should get rid of it.
Bluetooth: I toggle Bluetooth via an AppleScript I run via LaunchBar so it’s just there to indicate Bluetooth is turned off as I usually don’t need it except for very rarely tethering stealthily to my iPhone (without a dock cable).
iStat Menus temperature: Of my GPU (right) and CPU (left) and many more sensors in the menu when opened.
AppleScript: Rarely used, I maybe should get rid of it.
iSync: Actually only used to access the “Sync conflicts” dialog when I need to. I don’t sync with MobileMe as I don’t trust my data to it.
Keychain: My indicator to make sure all my keychains are locked when I have to leave my machine. (I use the keyboard to actually lock my screen whenever I have to leave my screen, even if only for a moment.)
ClamXav: Open source antivirus to check the occasional download for malware so I don’t accidentally send something infected on to some poor Windows soul. Should catch the few Mac native malwares as well as macro nasties.
: Only to restart Growl when it has gone wonky again. I should be able to get rid of that one as the last update has fixed a lot of instabilities. Notifications themselves are to be kept at a minimum and for emergency information only.
Espionage: “Encrypts folders” by putting their contents into a .sparsebundle and automounting said image in place of the folder. Makes selectively encrypting data comfortable.
MacFusion 2: Simple GUI frontend to  comfortably access remote filesystems via SSH or to mount FTP servers with write support in the Finder (which it still doesn’t do natively for no apparent reason).
Visor: The ultimate accessory to access a Terminal in the blink of an eye. (Yeah I know, one shouldn’t blink at any give time.)
SMARTReporter: Displays the S.M.A.R.T. status of all internal disks and goes red alert if a disk thing it might go bad in the not too distant future. S.M.A.R.T. is never guaranteed to tell you about impending disk failures. So please always have a least one good and current backup! Better more than that!
gfxCardStatus: Allows me to manually switch from integrated to dedicated graphics on my MacBook Pro to get some additional battery life when on the road (and I don’t need the graphics power).
Keyboard Maestro: To remap a few keys and have instant triggers for a few things like being able to control my iTunes volume via the volume keys (and being able to set the system volume with the same keys separately). Comes in very handy when using iTunes with AirPlay speakers. Haven’t yet found many other use cases for me yet.
: The Mac server part to watch movies from my MacBook Pro on my iPad via the corresponding iOS AirVideo client app for iPhone, iPod touch and iPad (Universal). Does on-the-fly transcoding of video files that are in a format which cannot be played back directly on iOS devices.
TextExpander: The swiss army knife of text input manipulation. Saves me countless hours of typing action for boilerplate foo and also does a lot of practical stuff like URL shortening with j.mp for twitter or entering obscure Unicode characters. (Disclosure: I am the author of these free TextExpander snippets.)
Dropbox: For occasionally sharing a few files with the family. I don’t use it to sync files across my own Macs.
Dropbox: Another Dropbox account for the austrian chapters of Cocoaheads, the international Mac and iOS developer’s community. To better distinguish between them I use one icon in monochrome and one in colour.
SizeUp: Window manipulation with the keyboard to which I am seeking an alternative as it doesn’t allow me to assign the keyboard shortcuts I want to have. (Mainly using the fn key which is not an option with SizeUp.)
Degrees: A simple display of the current outside temperature and weather conditions. Very handy when working in the data center catacombs where you don’t have a window.
AeroFS: A privately synched filesystem not unlike Dropbox but without the cloud server to keep your data a little more confidenial than Dropbox. (Currently in private beta.)
OmniFocus: Shows the due and overdue tasks of my favorite todo management application. (Please don’t ask about the count…) Also available as OmniFocus for iPhone and Omnifocus for iPad which perfectly sync with each other over my private WebDAV server (or other ways if you prefer).
(From , Mac OS X Server systems administrator and iOS developer)