If you work in Photoshop, you've probably had designs which branched off from the original look, based on your own ideas or client feedback. This has, more than likely, resulted in multiple files with names like mockup1.psd, mockup1b.psd, mockup2.psd, etc. The solution would be what coders know as Version Control, something which allows you to "commit" any version of your project to a repository, letting you jump around from version to version, making changes without worrying about losing one of those branches you went off on as an experiment. Version control systems like Subversion and Git can do this for you, but there's a learning curve and a certain amount of geekery required to make it work smoothly.
Timeline from PixelNovel aims to take the "geek" requirements out of using version control in Photoshop. Timeline goes beyond other options and uses a Subversion repository to store any revision of your progress while working on a Photoshop file. If you don't know what Subversion is, that's ok. Timeline simply shows you a row of previews of versions you've chosen to save, and you can jump back in time with a couple of clicks. Make a change, commit it, jump to a different version ... all in a slick, minimal toolbar. You can add comments (a commit message for those already svn-savvy) to each revision, which is handy whether you're working alone or collaborating.
Timeline offers two ways of handling the repositories necessary for it to function. The least geeky option is to use their hosted Subversion service, which provides easy-to-use collaboration options, a web interface and automatic backup. It's available at a subscription fee, but signing up includes a copy of the plugin for free. Alternatively, you can host your own Subversion repository, which isn't terribly difficult. This avoids both the hosting fee and (if you create a local repository on your own computer) the need to download your revisions from a host.
I spoke with the developers of Timeline about the potential for a Git version, and it's something they're looking into. For now, though, the Subversion plan is working quite well. If you frequently find yourself with a folder full of poorly-titled versions of a project, and flipping between them is getting to be a pain, it's definitely something to consider. The plugin itself will cost you $60US and -- if you choose the hosted route -- an account runs $5US for 5GB of storage, or $20US for 25GB. TUAW readers can take advantage of a special offer, and get 20% off of either: use coupon tuaw1 for a standalone license, and tuaw2 for a 20% discount on a hosted option.
[Side note: For the Git-inclined, I've found that the previews in GitX combined with a few shell scripts can provide a decent (but less integrated and robust) versioning system for Photoshop and other graphics applications.]