Whether or not you use the excellent Mac edition of this app, which was recently updated to version 3.0, 1Password touch is a great addition to your iPod touch or iPhone. Essentially a password manager, this app stores login usernames and passwords, wallet information like credit card numbers, software license keys, and secured notes behind one or two levels of protection (depending on how sensitive you deem the data). With cut and paste functionality, as well as a built-in web browser, it's a great way to simplify logging into password-protected sites or even making online purchases. Of course, to get the most out of the mobile version, a copy running on your Mac is a must. With it, you'll be able to sync your information between the devices.
The standard app is great and all, but since both versions are free for the time being, you might as well grab the Pro. Extra features include folders for better organization, the ability to copy multiple fields to paste elsewhere, and a unique switching mode to quickly copy and past info into Mobile Safari. In addition, there are some Pro-only features on the way including MobileMe and WebDAV syncing (I can't wait for that!), and Favorites.
If you already have the regular touch edition, make sure to back up your data before removing the old app and upgrading to the Pro version. Those with the Mac application can rely on Wi-Fi syncing to ensure the latest info is on the computer, however Agile has instructions on its site for those without. Once you've removed the standard 1Password touch and installed Pro, you can restore the data.