Things is probably one of the best task management applications for Mac that I've come across. And the only that I didn't completely forgot after 2-3 days. The iPhone/iPod app is just the cherry on top of the cake. You can sync with your Mac app and all.
The one feature Things is missing (besides being Mac only), is sync between different computers. I constantly use 2 Macs (personal and work) so this was a major turn off. But that's where Dropbox kicks in and saves the day.
Although not very explicit, Things has the option to select the location of your tasks database. So it's pretty much obvious: save the database in a Dropbox folder, configure all your Things apps to use the new database location and you're done.
This is where you're Things database is stored:
~/Library/Application Support/Cultured Code/
Just copy the
Things/ folder that lives in the path above to your Dropbox and then open the Things app pressing simultaneously the option key (alt). A popup will show up asking you to choose the library (database) location:
Now choose the Things folder in your Dropbox and that's it. Repeat this step for every application that you use on a different Mac, and you're ready to kick some serious ass at Getting Things Done.
You can find this info about selecting Things library/database on the Cultured Code wiki.
database.xmlfile on startup, if you don't close the application, it won't be aware of the new changes made to the database, and will eventually save the current state over the changes you've made in other instances of the application.