In OS X it is easy to create as many user accounts as you need. It's definitely useful being able to have an account for each family member, or an account just for troubleshooting. However, this also leads to an extremely long list on the login window, and an annoying scroll bar down the side.

If you are running Tiger, hiding user accounts that you rarely use is simple. Firstly, go to the Accounts pane in System Preferences, and find the "short name" of each user you want to hide. Once you have these, open up Terminal (Applications/Utilities) and enter the following:
sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add shortname1 shortname2 shortname3

Obviously shortname1 2 and 3 will be replaced with the short names of the user accounts you wish to hide. You can hide as many as you like, just separate each with a space.

To make a hidden name appear again, type the command with no names in it, therefore resetting the list of hidden users.
sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add

Notice that doing this adds an extra option to your login window - "Other..." When you select this, you will be presented with text boxes to enter a username and password.

If you aren't running OS X 10.4, another option is to completely get rid of this list of users. Go to the Accounts pane in System Preferences, and click on Login Options. There you will find an option to display the login window as name and password fields. This isn't as pretty, but saves time if you have lots of users, and is also more secure.

Here's a quick fun tip that I found over at macresearch.org.

• Make sure you have a cool screen saver set (macresearch suggests the RSS visualizer)
• Open Terminal (found in Applications/Utilities)
• Type the following:
  /System/Library/Frameworks/ScreenSaver.framework/Resources/
ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background
• Look at your Desktop.
• To stop the whole thing and return your desktop to normal press Control-C (or Command-.)

There are a few things to look out for with this: Depending on which screen saver you are running and what mac you are running it on, this make take up a large CPU load. Also, your normal background will be shown when pressing F9 or F10 to activate exposé. Finally, if your screen saver starts (from a hot corner or by inactivity for a certain time) it will cancel the screen saver on your desktop.

Many Mac OS X applications allow rectangular selections. That is, selecting multiple lines of text, without having to select up to the end of each line. This can be useful if you have created a text formatted table, and want to select an entire column, or if you want to modify the same thing on every line in a document.

To do this type of selection, hold down the Option (alt) Key while the cursor is over a block of text in a cocoa application. Notice that the mouse arrow changes to a cross.

With the Option Key still held, click and drag to make a selection without the text up to the end of the line being automatically selected when you drag down over multiple lines.

If you want to do something particularly neat, once you have made a selection hold down the Command Key as well as the Option Key, which will allow you to make another selection somewhere else in the document, whilst keeping the first block of text still selected.

This works just about everywhere you are able to edit text, including Microsoft Word (although the cursor doesn't change to a cross) and text boxes in Safari. As you would expect, it works in TextEdit, Mail, and all the other usual Cocoa Apps.