One of my favorite web sites has released a free app for the iPad. The Mashable app is essentially a wrapper for the Mashable site. It downloads the HTML stories from Mashable and caches them. This allows you to quickly and easily browse stories and topics. Here is what the app looks like in portrait.
Clicking on the Navigation button brings up a popup list of site categories.
Of course if you put the app in portrait mode, the list of categories is always displayed.