OneShot lets you highlight screenshots and post them to Twitter.
In order to use OneShot take a screenshot (by pressing the "home" button and the "sleep" button on your iPhone at the same time) in any app. Then open up OneShot and select the screenshot you just took.
You can then:
1) crop it
2) highlight text
3) change the background
4) select the source which should be automatically detected from the image
You can tweet the final image or save it to your camera roll.