Crazy Eddie's GUI System: This will integrate nicely with Ogre. It looks pretty too.
TinyXML: I decided I didn't want to write a buggy text parser myself for saving/loading games and enviroment maps.
OIS: Ogre doesn't really have an input system. OIS supports joysticks, which is something I'd like to have (time permitting).
I'm going to try and get the game design finalized and finish up the script by Saturday so I can start designing the engine. I'm trying to work a car chase into my storyline, but I don't know if it will fit in anywhere.