Mac Tricks

Terminology

Option key: Mac speak for "Alt"
Command key: The Apple key, used for menu commands.
Aqua: The name of the Mac OS X GUI. Meaning: You can usually substitute "GUI" for "Aqua", sometimes also used to differentiate X11-based programs from true Mac OS X programs.
Cocoa: An object-oriented API for writing Mac OS X programs, inherited from the NeXT operating system.
Carbon: A procedural API, inherited from the old Mac OS.
Universal Binaries: Applications (see below under "File System") can contain binaries for both the PowerPC processor architecture and hte Intel processor architecture. Applying File -> Get Info to an application shows it as "Kind: Application", followed by whether the application is Universal, Intel-only or PowerPC-only.
Rosetta: A just-in-time compiler that translates PowerPC programs to Intel code. Thus, most of the older Mac OS X programs still work, even if they are not Universal.

In application "NetInfo Manager" change the properties of /users/<your-login>: "home" or "shell"

Applications: Mac applications are just directories whose name ends with ".app", in the Finder, you can use the context menu to look inside such a folder. Advantages of this mechanism: Applications can be so-called "fat binaries" and contain code for multiple processor architectures and they are easy to install and uninstall: You just copy them wherever you want to use them and thrash them for deinstallation.

File system layout: Inspired by the Unix /usr/bin etc., but a bit easier to understand

Hidden files: Under the Finder (but not under the shell) much of Mac OS X's Unix personality is hidden.

Use an Option click to position the cursor in the terminal, e.g. when you are using emacs over ssh.

Open files with Mac applications: A nice side effect is that, if you are using ssh-agent, the applications will inherit its access rights.

Drag and Drop and Option-Tab: You can use Option-Tab during D&D, in order to easily transfer data between programs
Drag and Drop files on Mail.app: Creates a new email message whose attachment is the file.

Apple's apps store their preferences quite cleverly in a tree that can be accessed from the command line. Thus, the following hacks are possible.

iTunes: Making the arrows transport you to your library and not to the store. Option-Click will still take you to the store.

In the terminal type: "defaults write com.apple.iTunes invertStoreLinks -bool YES"

A technical introduction to Mac OS X: Mac OS X 10.4 Tiger by John Siracusa on Arstechnica.com.