There are some Unicode characters which are useful (and fun) to be able to type. However, while some of them are readily accessible using the keyboard, most are hidden away in the Character Viewer. Here’s a method which allows you to use the actual Unicode codes to type these special symbols.
Most Mac users know about the characters which can be accessed by holding down the Option key, such as accented characters using Option + E, and some others such as the Apple symbol — ? — using Option + Shift + K. But there are a whole list of other Unicode symbols which can’t normally be accessed with keyboard shortcuts. Normally, you’d open System Preferences, go to Language and Text, enable Character and Keyboard Viewer and access them that way. However, it’s even easier to enable Unicode input, which lets you type the Unicode code for a symbol.
Load up Language and Text in System Preferences, then choose the Input Sources tab. There’s a list of languages on the left. Scroll down the list and check the box next to Unicode Hex Input. You should see a small flag appear in your menu bar, representing the current input language of your keyboard. Click that flag and choose Unicode Hex Input from the list. It’s now possible to type any Unicode character that you know the code for.
To type a character, hold down the Option key, then type in the code for that character. For example, a pencil icon has the code 270E, so you would hold Option and type 2 7 0 E on your keyboard, and a pencil icon will show up, like this one: ?. For a full list of Unicode characters, you can visit this Wikipedia page.
Obviously it would be difficult to remember the codes for a lot of symbols, but if there are a few you have to use often, I’d recommend doing this instead of resorting to the Character Viewer.