I’ve been pretty happy using Things for the past several months. I’m not hardcore into the GTD crap, but I’m in the habit now of writing literally everything down and organizing it into different projects.
Not too long ago, I bought a desktop and needed a way to sync Things with my laptop. Luckily, I came across Dropbox which automagically syncs and shares files potentially across multiple computers (mac and pc) and they’ve got a really good screencast explaining the concept.
You basically just need to move your Things database into your Dropbox directory and then symlink it from there to your Things Library directory.
So something like this:
mv ~/Library/Application Support/Cultured Code/Things ~/Dropbox/Library
cd ~/Library/Application Support/Cultured Code/
ln -s ~/Dropbox/Library/Things Things