I have several workspaces, each dedicated to a different task. Each workspace contains multiple projects that are in some ways related (conceptually, if not physically). For example, I have one workspace for quick demo apps; there is no relationship between any of the apps. I have another workspace for the sample code for
JBoss in Action. I have several workspaces where I keep work-related projects for. Etc.
In some cases I have multiple Eclipse installations, usually because I want to use different groups of plugins.
I also tend to export my Eclipse preferences so that I can import them into another workspace since the preferences are workspace specific.