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.]
I came across this by pure accident but did you know that all of your iOS Keyboard Shortcuts that you create are then stored to iCloud Keychain and can be used in OS X 10.9?
I was in iMessage when I accidentally typed "eag" which is a shortcut I have created to easily insert my work email in iOS. Well I typed that and pressed space and the text autofilled into the message. So then I went ahead and typed other shortcuts I actively use in iOS and sure enough, they all auto-complete very nicely. After noticing this, I went to System Preferences » Keyboard » Text and found an entire Menu that you can control this (and I assume will also sync to iCloud Keychain).
[crarko adds: I'm pretty sure I've seen this mentioned before, but in case it is a new and useful fact to somebody here it is.]
I don't like paper-like background of OS X Notes.app, and the fact that, usually after syncing with its iOS counterpart, the application doesn't remember the larger font size which I've set. Here's how I fixed it to my satisfaction.
First, navigate to /Applications and Control+click on the Notes.app icon. In contextual menu, select the Show Package Contents item. Then, browse the content of the app's package to Resources, and inside that find these two files: pad.css and paper.tiff. Make a backup copy of these files in a safe place in case you want to revert back to them sometime; otherwise you'd have to reinstall OS X to do so.
Copy the former into a folder you have write permission for, and open it with any text editor. Just replace the @[FONT_SIZE] on fifteenth line with whatever font size suits you. I've bad eyes and have choosen 18px. Save the file and replace the original with this modified version.
Then, find or create a tiff image of the color you'd to replace the notes background image with. By the way, I think you could you any picture. I just used Capture to capture a plain white portion of my screen. Be sure this image file you produce is in tiff format. If not, you can easily convert it with Preview. Name it paper.tiff and replace the original file.
Restart Notes.app and changes should be there.
[crarko adds: I haven't tried this but as a general rule of thumb I'd make a backup of the entire application, as well as the files to be modified. Also it's possible that a Software Update could overwrite your changes, so just be prepared.]
I recently switched from working in plain text files to working in rich text in TextEdit and one of the most annoying things about rich text is pasting from other types of windows. I like to use a fixed width font most of the time, such as Courier New. Unless I'm diligent and use Command+Option+Shift+V, I end up with a mix of fonts and font styles. I finally got fed up enough with it today to do something about it. This work-around will only work with pasting from the keyboard. It will not correctly paste text that has been dragged and dropped.
The solution is to use Keyboard Shortcuts to switch the Edit menu's items for Paste and Paste and Match Style. It's such a simple fix, I never thought to do it before now. After this fix, Command+V will paste and match style and Command+Option+Shift+V will paste (and keep the style the copied text was in).
Open System Preferences
Click the Shortcuts tab
Click the "+" button
For Menu Title, enter exactly: Paste
For keyboard shortcut, type a Command+V
Click the "+" button
For Menu Title, enter exactly: Paste and Match Style
For keyboard shortcut, type a Command+Option+Shift+V
There you go. Now you can paste with Command+V and always match the style of the surrounding text.
[crarko adds: Simple, but useful. This kind of thing is applicable to pretty much any application. Do most folks here look into customizing their UI to suit their own work habits? For me editing this site depends heavily on customizations in (and around) BBEdit.]
Printopia is a great tool for exposing non-Air Print printers on a local network so that you can print to them from any iOS device. But even better is that it can expose PDF Workflow to do your bidding. In my case, I wanted to do n-up printing (n pages per sheet).
To do this, open up Automator and create a Print Plugin. For the workflow, you only need to add a single Run Shell Script action with the following line:
The details of the command will vary depending on your needs. In my case, I wanted 2-up printing to go do my default printer, so I could exclude the -d option.
To see a list of available print queues, type lpstat -a in terminal.
Once you save your Print Plugin, go to the Printopia in System Preferences and add your new plugin as an available destination for printing.
[crarko adds: I admit to being a happy Printopia user, and know there are others around here as well. While not needing this particular item myself, it does suggest some other things to try. Which is why I think it's a useful hint.]