Ever thought that Photo Booth doesn't provide enough image effects? Probably not; there are sixteen (very cool) effects built in. However, there are a couple of ways to get more effects. You could use this hint to add more effects directly to Photo Booth. If you'd rather not do that, though, try these alternatives.
If you want to make a new image, first open Automator and make a blank workflow. Drag the Take Video Snapshot action from the Photos library into your workflow (strangely, Take Picture does not work). Choose your file name and location, and select Take Picture Automatically, then run the workflow. Click the effects button next to the snapshot button, and choose your effect. There are twenty-nine effects there that aren't in Photo Booth, including Blur, Pixelate, and City Lights.
Alternatively, if you want to simply modify an image and don't care too much about size, here's another way: Open Address Book and create a new contact. While in Edit mode, drag your image into the picture well. Then, choose effects again and go to town. When you're done, drag it back out. You can apply more effects by dragging in again, choosing another effect, and dragging out. Of course, this can also be used to apply multiple effects to existing contact photos.