I'm using Keynote '09 for the first time, and I find myself wanting some bits of clip art or additional shapes. At the moment, I just want a big check mark. It's hard to believe that this isn't provided, but I can't find it.
Keynote doesn't really come with clip art. It is easy, however, to add images.
I did a Google Images search for
check mark, and set the color to green. Here are the results (there are many good images).
To add the image to Keynote, click See Full Size Image and click-and-drag it to your slide.
Add a text box. Start typing. Then press CTRL+CMD+space to bring up the characters popover. At the bottom of that popover scroll to the right until you see the ✻ category. Inside there you'll find a couple of different check mark/tick characters like ✓ and ✔︎. Once the symbol is in your document, resize it and choose a colour.