Hi MaddScientist,
I use eBay's own Selling Manager. eBay have also recently made this tool free in the UK
Unfortunately the tool still demands a monthly fee in the US.
Regardless of the cost, Selling Manager and Selling Manager Pro are both an awesome service. What's great is that the Selling tab from your My eBay turns into a Selling Manager tab giving you ALL the functionality of the service.
It's great as everything is fully integrated into your own My eBay.
If you head over to my you'll find links to the various services.
I hope this helps!
Take care,