One of the benefits of having the full Safari browser on your iPhone is that bookmarklets will work.  For anyone who hasn't heard of them before, they're little chunks of javascript that you save as bookmarks and which, rather than link to a site, perform handy little tasks to make your browsing easier.  LifeClever's Chanpory has put together a list of his favourite seventeen, including text searching and a dictionary.

There are plenty on offer out there, so use this list as a starting point and then go find (or code, if you're that way inclined) your perfect browser add-on.  Make sure to let us know in the comments if you find something particularly useful!