Although many people say Vista's ReadyBoost - uses portable massive flash storage as a drive for disk cache - doesn't improve performance I was interested to see a private company had come up with a similar solution for XP.
The eBoostr software allows you to use a USB drive as a memory cache. As a number of people have commented, for the price of the software ($39) plus the cost of the USB drive you could get on board memory for cheaper. However, I might have a use for this: In the past some of my clients have been reluctant to supply PCs with enough memory to run a full development environment (e.g. Java + Eclipse + JBoss + MS Office) , so instead of making an issue of it a nice solution would be to install eBoostr :)
Speaking of USB drives, I'm tempted to try out MojoPac which allows you to carry around your own XP data files, applications and environment on a USB drive :)