ive been using my ipad for a couple of months now and I have been trying out a boatload of apps. im using it like a replacement for my laptop, not for anything that needs the power of my macbook pro, but for most lightweight tasks it is perfect.
here is my list of most usefull apps
it lets you post to your blog, it lets you manage your existing posts and comments. one of my most used apps, and its free.
it lets you read all your rss subscriptions. its not free, but it is really good and syncs with google reader and lets you read your feeds even if you are offline :)
it lets you read all your tweets. its free, and its one of the best apps for the ipad. a must have.
like google earth, but upside down, it looks away from the planet and in to space.
there are thousands more apps, but these are the ones I use most often that dont come with the ipad (like safari, mail.app, calendar, etc)