Save and Load Workspaces
In practice, having a user account for each client actually works very well. The keychain contains items just for that client and you aren't distracted by your work when you are working on client's items. It's all about everything you need for that client.

Between webmail and web project management - you might even be able to keep some critical items out of the user account and be able to access it from multiple places.

I find that iOS is so good on managing the things I need, that I'm using "my" stuff on a computer so much less that one account per big project and a catch all consulting account to keep my personal account clear of work and be very nice for automated billing, crystal focus on the task at hand as well as clean separation of files and configurations where I can put things in a shared folder, but need to take a step rather than having everything all jumbled.

Fast User switching also helps to pop back to another account quickly when needed.