Sometimes you take tons of pictures only to discover that they're all wrong in the same way. Maybe the lighting was bad, or maybe you need to tweak the temperature slightly. iPhoto provides a really simple way to adjust one image and then apply those same changes to as many other photos as you want afterward. To access this groovy little feature, first go into Edit mode by selecting an image and clicking the Edit button (or by hitting Command-E).
Once you're there, make all the changes you want. Go nuts. Enhance the image. Jack the saturation way up. Move the temperature slider all the way to one side.
I may be missing the point of editing images, here.
Anyhow, once you're finished with your work, either press Option-Command-C (Edit> Copy Adjustments) or right-click on the photo and pick the same thing from the contextual menu.
Then enter Edit mode for the next photo you'd like to make changes to and press Option-Command-V (Edit> Paste Adjustments) or use the same contextual menu as before. That's a lightning-fast way to brighten up a bunch of photos from the same overcast day!
For an even more efficient way to apply changes, select up to four photos at once by holding down the Command key as you click on each, and then press Command-E to enter that same Edit mode. The difference here is that you can click each one in turn and press Option-Command-V to paste those adjustments, so you don't have to open each one separately.
Note that copying and pasting adjustments won't apply to any of the effects iPhoto can use (such as adding a sepia tone or a vignette), so don't go expecting that you'll make a bunch of photos all fancified at once.
It also won't work with anything that's very specific to one image, like the Crop tool. Otherwise, though, you can do whatever you want. Pro tip! I find that cranking up the exposure to uncanny-valley levels means that any wrinkles you have will be lost in the gorgeous nothingness that is your face.
That looks totally good, right?