Western Digital My Passport Essential 1TB USB 2.0 Portable External Hard Driveand I find it to be nothing less than awesome. 1 terabyte of storage is ample for backups of demos and presentations, and also a full backup of working documents. The drive also has encryption, and comes with easy utilities for backup and retrieval. It is also so tiny I am never without it on the road.
I have also looked into using eSATA drives for demos from external media - as the USB2.0 is a fair bit slower, but find the need for external power a bit too much for me right now. It could be good if you find yourself doing more webinars though.