A friend just pointed me at a very cheap software bundle for Macs that includes Scrivener, which has a fairly good reputation as a writing tool.

http://macheist.com/bundle/t/610045

No idea how long the offer is on, or which versions of Mac are supported, but I'm trying to find out.

Turns out the offer has nine days to go.

Got it tonight - basically, all of the applications are for Intel-based macs, but you can download the "legacy" version of Scrivener from their site and use the activation code with that, and it runs fine on my old iBook. The games come from Steam, which means that there are Windows versions as well as Mac; I've installed a couple on my Windows box without problems.