PowerPoint comes with dozens of free shapes; circles, stars and even smiley faces are only a few clicks away. If you don't find a shape you like, draw your own using the built-in free-form drawing ...

PowerPoint 2007 has WordArt tools that let you generate text not only in a half-circle, but in all sorts of rings, waves, slants and fades so you can morph your words into whatever shape you envision.