Articles [$2.99 - iTunes link] is a thoughtful, elegant new Wikipedia reader and discovery tool from Sophiestication Software.
Rather than just displaying the Wikipedia.com page, Articles takes the data behind the page and reformats it in a very clean, easy-to-read format with a chapter index control to briskly skim your way through to the section(s) you want to focus on. You can search for articles, find articles based on your current location, or ask Articles to surprise you.
Two of the more interesting features are Mobile Safari-style "Pages" that allow you to keep several articles open at the same time and quickly move between them, and an interesting variation on the pull-page-down-release behavior that, rather then refreshing, toggle between orientation lock and unlock (so your iPhone won't spin content around wildly if you're reading in bed, for example).
There's also Mobile Safari-like bookmarks and history, and additional tips include double-tap-and-hold to display the chapter index, pinch or stretch to decrease/increase text size, tap-and-hold links to open articles in a new page.
Is it worth $2.99 when wikipedia.com (and other Wikipedia-based apps) are free? If you want more than the raw data, if you like the experience and features Articles provides, then absolutely. I read Wikipedia a lot and Articles is the way I'll be reading it for the foreseeable future. (They had me at multiple pages).
Screen shots after the break, and if you give it a try, let us know what you think!