Workspaces are designed to increase the productivity in Ubuntu, presented as "separate" desktops accessible via keyboard shortcuts (in Ubuntu 11.10, press&hold Ctrl+Alt then navigate between workspaces with the arrow keys, like Up, Down, Left, Right) or by pressing the Workspace Switcher from the Unity launcher.
There are situations when users want a faster way to switch between workspaces without pressing keyboard shortcuts or by using extra mouse "effort", faster way that can be easily enabled via Ubuntu Tweak.
The usage is simplistic, open
Ubuntu Tweak-->Tweaks-->Compiz Settings-->check Enable workspace switching with mouse wheel.
The resulted behavior is: switching workspaces (horizontal only) is done now via mouse wheel, furthermore the switching action is applied only when the mouse is positioned outside of the focused/unfocused window (on the actual "empty" desktop), thus removing the accidental actions.
The workspace number can be adjusted (both vertically and horizontally) via CompizConfig Settings Manager under
CompizConfig Settings Manager-->General Options-->Desktop Size-->Horizontal Virtual Size (available via Ubuntu Software Center) or MyUnity.
Running MyUnity, the number can be increased under
desktop-->H Desktop (choosing the number 5 will create five horizontal workspaces usable under the above scrolling "method").