The main index is simple yet elegant; on the left side of the page is a purple (or green for all I know since I'm colorblind) area with links for such topics as "Improving Your Writing Style," "Stages of the Writing Process," "Common Writing Assignments," and "Grammar and Punctuation." All of these proceed onto further pages of links offering advice in sentence concision and comma usage, most of which offer solid examples.
What I found unique about UW-M's OWL is that one of the pages offers a "self-test and answers" link that allow you to "test your knowledge" of a given subject. Want to see how up to snuff you are at commas? Scroll to the bottom of the "Using Commas" page and you'll find a link that will initiate a pop-up with a few example sentences to see if you can correctly identify the issues in the sentences. Though most of the other pages lack this feature, it could be a feature that's "in the works" that I would love to see in a more complete form some day.
The Writer's Handbook also features a section on reference citation that, though it lacks the thoroughness of the OWL, still recognizes the other systems outside of MLA and APA. At first, I thought that the page on Chicago style is a bare-bone description noting that "Chicago or Turabian style places bibliographic citations at the bottom of a page or at the end of a paper." On the far right side, however, they offered links to learn more about the documentation style. Aesthetically, it was a little confusing as their MLA page has a clear "Table of Contents" in the center of the page while on the Chicago/Turabian page there is no "Table of Contents," just a side menu. For the unobservant, this menu will elude the reader and cause the reader to go to a different, easier to navigate OWL.
Though not perfect, The Writer's Handbook at The University of Wisconsin - Madison has the potential to grow into a pOWLerhouse (worse pun), offering up great ideas yet not as thoroughly executing them. Some pages are a bit roughly designed, but for the most part it's an impressive alternative to the Purdue OWL.