OK, so I've got a copy of Leopard. Problem is, I didn't want to just do an upgrade as I've read so many stories of blue screens, problems after installation, etc., and I didn't want to have to back up my data and do a clean install. So, I though the best thing to do would be to install Leopard on its own partition – but how to create a new partition without erasing all existing data? I had a look at iPartition which looks like a very good product, but I didn't want to have to pay for something (Leopard's expensive enough already). Anyway, to cut a long story short, I booted from the Leopard DVD, opened Disc Utility and selected my hard drive then clicked the 'Partition' tab. To my surprise, I saw that Disc Utility lets you create a new partition without erasing any existing data. Fantastic! It'll even let you resize an existing partition if necessary, and, as long as there's enough free disc space, won't erase anything.
There you have it – Disc Utility in Leopard lets you create and resize partitions without erasing data.