I find more than one window confusing. Within my one window/workspace dedicated to translation, I have tabs belonging to different projects. Even if I had a separate window/workspace for each project (and I work on them concurrently), within each project there are different aspects. For example:
Let's say I am working on the OpenOffice.org project alone, even though I always work on several projects concurrently. Within the OpenOffice.org project, I have opened status pages, wiki pages, website pages, sub-project pages, issue-reporting pages etc. The information on one of my wiki pages requires further information on several other wiki-pages related to it, which I then open. It would be much easier for me if I could open those additional tabs below the current wiki page tab, instead of having to move down to the bottom of the tab list to read them OR drag them up the tab list to the appropriate position.
I am constantly following links to additional information, which is often only necessary temporarily. It's not useful to set these tabs up in separate workspaces. It _is_ useful to be able to open them next to the originating tab, so I can switch between them quickly, and don't get them confused with other tabs.
For example, within one individual Issue Tracker, I can be following up several separate issues at any one time. Each separate issue will have related issues, webpages, wiki pages and mailing list archive items, which I need to keep open next to the main issue tab, and not confuse with other issue tabs, while I am dealing with the issue.
For both these sample situations, the ability to open a related tab directly below the current tab, and the ability to collapse groups of related tabs, would be very useful indeed.