Reveal Hidden Menu Bar Icon Functions in Snow Leopard
In Mac OS X 10.5, the AirPort icon in the menu bar had a trick up its sleeve--hold down Option when clicking the icon, and you'd see more information about the various available AirPort networks. In Snow Leopard, this feature has been expanded, not just for the AirPort menu, but for some others as well.
Here's what you'll see when you hold down the Option key and click on various menu bar icons:
AirPort: In addition to the hardware (MAC) address of the wireless station, which channel is in use, the signal strength (RSSI), and finally an indication of the data transmission rate (all available in 10.5), you can also see what mode your connected network is using (802.11n, for instance), security (WPA2 Personal, etc.), and something called the MCS Index, which appears to be related to the 802.11n standard. (If you're really into this stuff, feel free to look up your MCS Index value on this table; interpreting what you find there, though, well, that's up to you!).
Sound: If you use more than one sound input and/or output on your Mac, you'll love this change. Instead of using a third-party app such as SoundSrouce, or opening System Preferences, just Option-click on the Sound icon. Instead of the usual volume slider, you'll see a list of available output and input devices; just select one from the list to use that output or input device. This is a great timesaver.
Battery: When you Option-click on the Battery icon, you'll see one of Normal, Replace Soon, Replace Now, or Service Battery right next to the word Condition. Click this entry, and an Apple Help page will open, explaining what each term means--if they're not sufficiently self-explanatory already. Apple's also got a support document that explains more about battery condition. While the cynics may be thinking this is a good way for Apple to sell more batteries, with non-removable batteries becoming the norm, more information about their condition is a good thing.
Sync: Option-click the Sync icon in 10.5, and you won't see anything different than you would with a normal click. Option click it in 10.6, though, and you'll see detailed information about exactly when each of the data types you sync was last synchronized--and this list includes any third-party application that use Sync services, such as Transmit and TextExpander. You'll also see an Open iSync Diagnostics menu item, to help troubleshoot any sync issues you may be having.
As best as I can tell, those are the four icons with new Option key features in Snow Leopard. Other icons either have no Option key features, or (as with Bluetooth) have the same features they had in Snow Leopard. Please feel free to correct me if you find additional new functionality, though.