Evernote is one of the best ways to keep track of everything you're doing or anything that catches your eye. The app works best if you take advantage of it's desktop equivalent as well, considering the two services sync up almost instantly. So instead of e-mailing over a text only description of the new t-shirt you want, you can simply access your Evernote app on your iPhone and see the whole section you highlighted with the picture in it already.
On your computer, one of that main functions of Evernote is that it allows you to highlight a specific area of a site and add it to your notes. On the iPhone, not only can you view these notes that you've created previously, you can also make new notes through four different methods: Text, Snapshot, Saved Photo, or Audio. Every option allows you to input the name of the note and even tags for the note itself so that you can more easily find it at a later point. Once you've finished the note, it enters the pending area where it syncs with your online account, making it available on both machines at the same time in the notes folder.
Overall, the app runs very smooth. The only issue that seems to be consistent is that some notes can get stuck in pending for quite awhile if you close the app without first letting it sync up. There isn't really any way to force the note to try and send again, nor does the note save a copy of itself anywhere else, forcing you to just wait it out while sitting on the pending screen. This is more of an issue with pictures, but is something that can be overlooked as every other service that allows you to transfer a picture has taken around the same amount of time to upload. Considering few people with an iPhone would be sitting at their computer without it, this isn't really an issue.
Evernote is one of the few must have apps for the iPhone, even just as a replacement for the Notes app that comes standard with the iPhone.