One that impressed me was a product called "Doceri". This one is a little different because it does a whole range of things.
First of all - it mirrors your laptop for Interactive Whiteboard presentations. From your ipad you can control your laptop and any presentation to your Interactive Whiteboard. For example - it may be as simple as a Keynote presentation that you control from the back of the room. Or it may be the Smartboard software and a lesson you have prepared. Instead of asking the student to come to the front and manipulate the smartboard - take the ipad to where the student is sitting, hand the ipad to them and ask them to do the same thing. Great for students who are a little hesitant or shy. Anything that can appear on your laptop can be used through Doceri on your ipad.
That isn't the only thing it will do. You can also write, annotate over any program on your computer. Anything on your computer (including interactive software) you can control (mac or windows), easily launch any document and annotate over them. You can record and save any drawings or presentations for later use. If you just want to use it as a whiteboard - you can. Write or draw on it (or have your students do it). I have tried it in the classroom with students for Interactive whiteboard lessons and it works great. The shy students appreciate not being the centre of attention (just their work). I also used it for a ipad users group presentation which worked great. You can even use it to control projectors, DVD's and other audiovisuals (although I haven't tried that). All in all - a great and very useful program. You need to download it onto your computer. It costs $50 to buy but you can trial it for nothing (but a reminder comes up about every hour or so - that it is a trial.) But other than that - it is fully functional. The app for your ipad is free - from itunes. Here is the link. Give it a go - a very impressive product.