Quotebook is another "one thing well" app. Developed by Lickability it's tagline is "a notebook for your quotes" and that sums it up in it's entirety. At it's heart it's simply a basic database of quote text, sources and attributions. You can rate and tag your quotes for quicker browsing and searching but it's function is singular. It builds on this by integrating with social websites where you might wish to share your quotes such as Twitter or Tumblr, which has proven for me to be it's most useful feature. One of it's other nice features is it's Wikipedia and Wikiquote integration, allowing you to read about the person in question or find more quotes from the same source. It also recognises quotes in text from the clipboard for easy addition of new material. If you collect a lot of quotes then this is the app for you.
Also check out the video demonstration on the site (click play on the iPhone). I thought that was a pretty excellent touch.