Some of Apple’s OS X icons Think Different
The text of “Here’s to the crazy ones” is on the Text Edit icon:

Some of it also appears on the new All My Files icon in Lion:

Safari Reading List maintains scroll position when exiting, re-entering

If you are in the middle of reading a long article in Safari’s new Reader feature (View > Show Reader) and exit to see the original page, you will resume at the same part of the article upon reentering the Reader view.
Put your Mac to sleep with the Apple Remote
You can quickly sleep any Mac sporting an IR receiver (the MacBook Air, for example, does not have one) by holding down the Play/Pause button on an Apple Remote.
“Reply All” button in Mail.app
Sometimes when you need to fire off a quick e-mail reply to someone you hit the Reply button… quickly. As you compose your brilliant reply you realize that what you really wanted to do was “Reply To All”. Instead of forcing you to close your composition window and start over, Mail.app features a handy “Reply To All” button right in your message composition window toolbar. Simply click to add the recipients and carry on with your brilliance.
When adding multiple email addresses in Mail, you need to separate addresses with a comma and a space. After typing (or pasting) an address and then typing a comma, a space will be automatically added.
Type to search in long drop-down menus
Instead of scrolling through a long drop-down menu to find an entry (for example: picking a country or your birth year), just start typing and it will highlight what you type.
Target Disk Mode gets Thunderbolt icon, picky

A new MacBook Pro with Thunderbolt running in Target Disk Mode will display a new Thunderbolt icon along with the traditional FireWire icon. Once you plug in a cable for either connection, the other icon will disappear. Unplug the cable and the icon returns.
Photo courtesy of Patrick Scott
Get a sliver of volume
Ever wish there was a notch of volume between zero and one? Simply press the “volume down” key until there are no dots showing, and then hit the “mute” key.
The volume icon will display sound again. Even though the volume measurement bar remains blank, you should be able to hear the faintest of sound (you might need to plug in a pair of headphones).
This works at least in 10.6 Snow Leopard; not sure about Leopard or Tiger.
[Editor’s note: I verified this on my 2010 15-inch MacBook Pro. I’m not sure how useful it’ll be, but it’s there for people who want it.]
Even if you quit the Mac App Store, app downloads continue
Once the an app flies down to your dock and the download progress bar begins, you can quit the Mac App Store app and the download and installation processes are not affected.