On April Fools' Day each year in the past, we have given you a hat.
This year, you can choose your hat! You can also choose from a variety of other flair for your Ravatar. To do this, go to your profile page by clicking your username in the upper right of the page.
Then, click on the "add some flair" button you see under your Ravatar. When you click on that you will see this menu.
You add flair by dragging the little icons onto your picture. If you decide you don't want something, just drag it back out. Click "I am finished" in the bottom right when you have it just how you want it.
Now your awesome flair-filled Ravatar will show on your profile page and when you post in the forums, but only for today.
Happy April Fools' Day!