You may be running out of disk space fast if you have small SSD start partition on OS X Lion if you leave MobileBackups enabled.
If you open Terminal and run the command "df -h" you might see the (otherwise hidden) directory .MobileBackups using up a lot of space.

This feature is described in the Ars Technica OS X Lion review for example. It is there so your laptop will run backups even when you're on the move with no backup disk attached.

To disable it, run the command "tmutil disablelocal" in Terminal.
To enable it, use "sudo tmutil enablelocal"

This MacBook (black 2 GHZ Intel Core Duo) would not accept the firmware password I just set via the Firmware Password Utility from the Recovery Partition in OS X Lion 10.7.2.

The following steps let me turn off the firmware password protection. It only works if you haven't forgotten the password you had set. I my case this means I had used the correct password but it wasn't accepted. Seems like a bug, maybe specific to my particular (and rather old) hardware.

1. Open Terminal and type diskutil list
2. Mount the Apple_Boot Recovery HD partition with diskutil mount disk0s3 (use the identifier you see listed)
3. In Terminal type: open /Volumes/Recovery\ HD/com.apple.recovery.boot/BaseSystem.dmg
4. In Terminal type: open /Volumes/Mac\ OS\ X\ Base\ System/Applications/Utilities/Firmware\ Password\ Utility.app
5. Now you can turn off firmware password - again, provided you know the password you had set

Create a directory, move your files into it, issue this command from terminal:
hdiutil create -encryption -stdinpass -srcfolder mysecretfiles mysectretfiles.dmg 

Delete the unencrypted directory you created to start with.

OS X seems to cache ssh-key passwords by default. This is rather useless. You can disable this with
launchctl unload /System/Library/LaunchAgents/org.openbsd.ssh-agent.plist

Source: http://www.reddit.com/r/apple/comments/9fnnc/ask_rapple_how_do_i_disable_sshagent/

If you launch Keychain Access (in /Applications/Utilities/), and remove the entry for your ssh keys, you should be prompted for your password, and have the option to not save it.

To ~remove ssh-agent you can could run..

launchctl unload /System/Library/LaunchAgents/org.openbsd.ssh-agent.plist

..or use http://lingon.sourceforge.net/

Or unset the SSH_AUTH_SOCK env variable

Or, look at the ssh-add command - ssh-add -D forgets all identities (including removing relevant entries from Keychain), ssh-agent -t 123 sets it to remember identities for 123 seconds

As crazy and esoteric it may seem, syncing your Apple AddressBook to Google really only works when your Mac is synchronized with an iPod or an iPhone. Just plugging such a device and syncing with iTunes triggers the sync to Google. You can also edit a file on your Mac instead to get the same result, as describe on Lifehacker.com:
Change the "Family ID." value to 10001 in ~/Library/Preferences/com.apple.iPod.plist

Thank you Gina Trappani and Josua Ferris!

mdworker (visible in Activity Monitor) is the Spotlight indexing process (active even though the process is not visible in the spotlight menu).

We had a situation where movie files from a mobile phone (file ending .3gp) caused problems, as the indexer didn't seem to get past them. Using up 99% and more percent CPU which caused the fan to go crazy. Removing the files resolved this CPU madness.

You may find which files could be the problem if in Activity Monitor you click on the mdworker process and the click "inspect" in the menu, then choose "Open Files and Ports".

Even in 10.5.6 Mail still won't place the signature above the quoted text in replies, even if said option is ticked in Mail preferences. The only solution I found so far works, but is sure isn't great:
greate a signature with nothing but hitting return once and check the "place above quoted.." and ake this the default signature.

Google offers an iPhone Google Calendar App to synchronize Google calendars and introduces CalDAV sync for iCal and Sunbird.

See here: http://www.google.com/support/calendar/bin/answer.py?answer=99355

and here: http://www.google.com/mobile/apple/sync.html

To acces The Common UNIX Printing System (CUPS) Administration on Mac OS X, type this address in your Web Browser:
http://127.0.0.1:631/

About CUPS

CUPS provides a portable printing layer for UNIX®-based operating systems. It is developed and maintained by Apple Inc. to promote a standard printing solution. CUPS is the standard printing system used on MacOS® X and most Linux® distributions.

CUPS uses the Internet Printing Protocol ("IPP") as the basis for managing print jobs and queues and adds network printer browsing and PostScript Printer Description ("PPD") based printing options to support real-world printing.

If you're booting into Bootcamp Windows on your Mac and can't get past the login Screen because your Keyboard won't do CTRL+ALT+DELETE, press the cmd (apple/command) key + u to call up the on-screen keyboard, where you can click the required keys with the mouse.

In Parallels, choose "Send Keys" from "Virtual Machine" Menu.