Something will go wrong. Assume the part that's going to go wrong is related to the most important part of your presentation (Internet connection, projector, etc).
(At least one) paper copy of your slides.
White board markers for drawing. Chalkboards often have a few nubs of chalk you can use.
Business cards.
Water or coffee.
The room number. It's a real pain to have to open your laptop to open your email program to find the email to see what room you're in. Trust me.