One of the benefits of using GNOME 3 are shell extensions. They provide the user with added functionality as well as removing/hiding annoying features. They can also completely change the default GNOME interface. Here are few that I personally find useful but more can be found at the GNOME Shell Extensions website.
Show Desktop from Overview
By default, getting to the desktop isn't as simple as you would think. With this extension you can click on any empty space while in overview and it will take you right to the desktop.
This extensions gives you the ability to customize the Activities text in the top left corner of the screen. It also lets you add a icon, change the hot corner sensitivity, and change the panel transparency.
No Topleft Hot Corner
Although you can render the hot corner inoperable with the extension above, if you simply want to disable the hot corner and nothing else this extension does the job.
This extension is incredibly useful if you use a laptop and don't have a dedicated brightness control button.
Drop Down Terminal
If you use the terminal frequently, this is an excellent extension. Pressing the ~ key (the above tab default) will toggle a drop down terminal with a simple keystroke.
If you don't use the accessibility button in the top panel, this extension will hide it.
Replaces the Alt-Tab and iterates through windows in a cover-flow manner -- it's much more visually appealing.
This extension changes really declutters the panel by replacing the username and status icon with a power icon.
Hide App Categories
This extension hides the app categories in overview.
When headphones are connected and are unplugged, any songs that are playing will be paused.
Insensitive Message Tray
More often than not, I trigger the message tray pop up at the bottom of the screen unintentionally.
Feel free to post your favorite GNOME extensions in the comments below!