Ubuntu has Fast user switching, as does any other distro that uses a recent version of Gnome, KDE may have implemented something similar as well.
However it sounds like what you really want to be using is workspaces (I think KDE may call them virtual desktops)
These have been with Unix/Linux since pretty much the dawn of the GUI and I think Apple have something called "spaces" which does a similar thing.
But essentially you have a number of desktop areas (usually 4 but it can be changed) which you can switch to and from easily using a keystroke or a small applet at the bottom of the screen called a pager, the pager can also show the window outlines of each screen so at a glance you can see which of your desktop areas are empty.
I and I suspect many others tend to use this in a very similar way to how you describe, so I will have things like email, irc and skype on one desktop, open office with a half written document on another, perhaps some long running copy operation or download running on a 3rd and so on.
Although I must admit I use this a lot less since having dual head (twin screens) because now I just tend to keep all communication related things on one screen (email,chat, skype etc) and the real work on the other. Generally now I only flick desktops to stop the chat etc distracting me.
The advantage this has for a single user over fast user switching is that applications can be quickly moved from once desktop to the other without losing their state (generally it is a right click option on the application taskbar or with gnome at least you can actually drag them around using the pager at the bottom of the screen) or it is even possible to set an application to be available by default on all of your desktops (can be handy for things like chat clients)
If you have an email client that can put a new message notification on the taskbar or panel then it will be visible on all desktops when a new message comes in
Also on any machine made this decade desktop/workspace switching should be as close as it matters to instant. Whereas fast user switching will at the very least dump you back at the login screen so you can select which user you are switching to.