The Secure Notes feature in Keychain Access now allows syncing of notes across multiple Macs using iCloud Keychain.
To add a note to be synchronized across all of your configured Macs, select the iCloud keychain from the list and select File » New Secure Note item.... Make a title for the note, input some content, and save it. This note will now appear across all Macs. This feature is still exclusive to Mac OS; there is no iOS support yet.
Another way to add a note to the iCloud keychain is to drag an existing Secure Note from another keychain onto the iCloud keychain icon in the keychain list. Option+drag to copy, or just move it with drag and drop without a modifier key.
[crarko adds: I didn't have a chance to test this yet. I hope Apple does add this to iOS as well. It would be nice to have things go the other way.]
In OS X 10.9 Mavericks a smart folder (or a saved search) dragged to the Dock behaves like a folder (smart folders by default are saved under ~/Library/Saved Searches). Right click gives sorting, display and viewing options similar to ordinary folders dragged to the Dock. A drawer icon is shown if Display as Folder option is selected. In grid view Quick Look works, too.
I'm not sure if this is documented already. When you create a secure note from Keychain, it will accept an image or movie just as easily as it will accept text. I was able to put an image in one note and a movie in another. You can also play the movie while it's secured inside the note.
Create a secure note from the menu bar, then copy and paste the image into the note. That's it, the image is saved inside a secure note. You're done.
To put a movie inside a secure note, go to the file location on the Finder to locate the movie. Drag the movie to the body of the secure note.
[crarko adds: I'm not sure if it's documented either. This is a fairly old submission in the queue, so things may have changed recently.]
As satisfying as the swipe to close feature in mobile Safari is, it becomes a bit of a chore to close more than a few tabs.
To close all tabs at once, tap the new tab icon (two overlapping squares), tap Private, and then Close All. Repeat the first two steps and tap '+' (or the screen) to get back to an empty Safari in your preferred browsing state.
I only have an iOS 7 device to test this in.
[crarko adds: I tested this, and it works as described. I also only have iOS 7.]
Maybe less of a hint, and more of a "I didn't know you could do that!"
If you have connected to a remote Mac using Screen Sharing and don't have a mounted disk, then you can still copy between the two Macs via drag and drop.
Basically, you take a file from the local Finder and drag it to a window of the remote Finder. This will automatically initiate a copy. The reverse is also true. A little experimenting shows numerous applications can act as source, but in all cases a Finder window needs to be the final destination.
[crarko adds: I've done this for ages using things other than Apple's built-in Screen Sharing program, so I don't know when this became available. Did it come along with AirDrop? The full Remote Desktop program has done this since the beginning.]
When re-installing Mountain Lion from the Recovery Partition, the installer needs to check installation eligibility with Apple's servers. If your computer needs to access the Internet through a proxy server for whatever reason, the installer won't pick up on this; it will attempt to make a direct connection, fail, and tell you to contact AppleCare.
As per my earlier hint (10.7: Get the Lion installer to work behind a proxy server), you could simply use the networksetup command in Terminal to get around this. However, as of 10.8 Apple's software download servers appear to require additional checks to verify machine eligibility which - if you're behind a squid proxy server - may require additional configuration changes to squid itself in order for it to work. You may therefore need help from your network administrator for that part.
[crarko adds: If someone still happens to be running Lion (10.7) can you try this and post about which delimiting symbol worked? The referenced Apple discussion implied the change happened with 10.8 but the original text for the hint said 10.7, and I'm curious to know which is correct. Thanks.]
I don't know how long this has existed (or what to call it), but in 10.9.1, just as one is able to quickly retype previous commands in Terminal by pressing the up arrow key, it is possible to retrieve a copy of entire, previously typed messages in Messages.app with a similar key command.
Just press Option+Up arrow while the cursor is in the message-typing box. How many previous entries it will resurrect appears to depend on how much of the message history is currently in memory. If you scroll further up to load more of the message history, those additional messages become available to the key command.
[crarko adds: I believe we called it the command buffer in Terminal.]