Version control systems built by and for developers can leave designers out in the cold. Here are some options for those whose work isn't based on text.
It's a cultural thing. Most designers I know won't go near a command line. And the default distributed VCS for web projects, git, is best approached from the command line. Oh, there are GUIs for all platforms, but most everyone agrees they are not as good, not as complete, and not as flexible as the command line. Git is just so relentlessly geeky (it was invented by the father of Linux, after all).
As git's usage grows, at the expense of older DVCS such as CVS and Subversion, GitHub is increasingly becoming the place where development teams work together in the cloud, and where individual developers work in public. But GitHub is not going to be of any use to a designer who is not working locally in git.
Specialized tools
There are online tools customized for the designer's workflow. Some do a piece of the work, such as sharing and getting approval on design mock-ups or delivering finished work to clients. Others facilitate the whole workflow of storing, tracking, versioning, getting reviews for, and delivering design work.
MockVault is specialized around getting approval for mockups. It supports annotations, discussions during review, and a formal approval process. MockVault stores multiple versions of mockup sets, but I couldn't find any sign of tools for comparing the versions, merging changes from one to the other, etc. There is a free tier of service -- one design, unlimited versions -- and paid service starts at $12 per month for 20 designs.
Influence may be the app to beat in the mockup/review space. Here is an overview of the Influence process. Its maker, ZURB, calls it "the best way to get clear feedback on your designs" and claims over 64,000 users. ReadWriteWeb calls it "sleek." Influence costs $19 per month and up.
Shipment initially tackled a different part of the design process: that of getting finished work to clients. See ReadWriteWeb's review from last spring. Shipment is built on Dropbox and is currently in closed beta. It looks like the product is branching out to encompass some form of design reviews and realtime notifications.
Soup to nuts
LayerVault seems to have the broadest ambitions of this bunch, letting you "store, track, review, and deliver your design work," according to (again) ReadWriteWeb. LayerVault has implemented its own Dropbox-like cloud storage and sharing functionality. The product stores multiple versions of a wide variety of file types, from .PSD and .AI to the common graphic formats to .GRAFFLE, and provides access to them via a visual timeline. There is a "Wormhole" tool that lets you examine any area of a design to see what changed from version to version. And, most interestingly from the point of view of VCS, there are "Outposts": integrations with GitHub, Dropbox, Basecamp (project management), and Campfire (realtime chat). LayerVault has no free tier; plans start at $19 per month, with a 30-day free trial.
The product space of designer tools is a lively one with many active developers vying to outdo one another. Designers will be the beneficiaries.