So far I’ve tested Dropbox syncing with the following apps:

• 1Password: Involves a few steps, but Agile has also written a great setup guide.
• VoodooPad: Just drag your .vpdoc files to your Dropbox and you’re all set.
• The Hit List: Just move the library file from “~/Library/Application Support/The Hit List/The Hit List Library.thllibrary” to your Dropbox, hold down the Option key while launching The Hit List and select the file on your Dropbox.
• Things: Setup is similar to The Hit List, a user-contributed how-to is also available.

Most other applications can probably also be synced using one of the methods above, provided they support a custom library location or use documents to store your data. I have yet to run into any issues, however there are a few things you’ll want to look out for, to preserve your data integrity:

• Try to avoid running your synced applications on both Macs at the same time – otherwise you might run into “lost updates” and other syncing conflicts
• Occasionally check your Dropbox folder for aforementioned conflicts: Dropbox doesn’t give you a lot of warning if there are conflicts, so take a look every now and then to see if there is a “sync conflict” folder in your Dropbox.
• Make sure you have the same version of the application on both Macs.
• Your preferences are usually stored in a different location than your data, so don’t be surprised if those aren’t synced

Tip: if you sign up for free using this referral link, you’ll get an extra 250MB storage.