If you have a Twitter account, Adobe Illustrator, and have any desire to make a visually appealing Twitter profile banner, then this post is for you.
This Illustrator template will provide you with the right dimensions for a Twitter banner (1500px by 421px). It'll also provide you with basic layout for what you would expect to see once it is live on Twitter.
Here is one I just cooked up (Homerchu, I choose you!):
..and in action:
You can download it here.
Credit to Twelve Skip for the template image and TheJesus for HomerChu.