iPhone and iPod Touch users have a fantastic new solution for keeping track of log-in credentials from site to site. It's called 1Password, and like the name suggests, you only need to remember one password to access and use your log-ins across hundreds and thousands of sites.
Like RoboForm (download) and other desktop password solutions, 1Password lets you save these log-ins under the protection of a single master password. Unfortunately, due to the limitations of Apple's SDK, you can't run 1Password while you're randomly browsing in Safari, meaning you won't be able to enjoy the ease of autofill. 1Password's workaround is to have you plug in your log-in information and the URL of where that log-in screen is located. From the application, you can simply click on the site you want to go to and it will plug all of that information into the correct fields when it opens in an in-app browser.
The application is already off to a great start, but there are some quirks that need fixing (and will be getting soon). The most glaring omission is the lack of an on-screen keyboard, meaning if there's something like a captcha or another form to fill in later on, you're out of luck. You're also unable to delete saved log-ins, so any log-in you no longer need must be repurposed instead.
Otherwise, there's a lot of power for advanced users. Once you're browsing any site, if you have to log in again for something, you can simply hit the "lock" key, which will plug in your username and password yet again. The same can be done for forms if you're willing to make a preset for that--something just fantastic when you need to enter billing information without killing your thumbs.
Mac users who want to carry over passwords from their browser can also take advantage of cross-platform sync, which will port over log-ins from their desktop to their phone and vice versa with the $35 desktop version.