Guides help you to line control objects and paths easily in Illustrator. Here's a short overview of the options and how Smart Guides can take it a step further.
The twirl tool is a great tool that can make complex and eye-catching objects in no time. Here's one way to use it.
Opacity Masks are all over the web. Here's a short tutorial on how they work in Illustrator and how you can use them in your designs.