Group windows in Mac OS X
I do believe I understand what you are requesting, and actually find it very interesting. However, this functionality does not exist in the base OS itself (10.5.x | 10.6.x | 10.7.x). An alternative to grouping windows in OS X, that you be interested in, is to utilize Mission Control (10.7) and Expose. In Mission Control you can add a new Desktop and drag your window(s)/Applications into that new Desktop thereby grouping them. They do not have to be like items and this can be done a number of times. Expose does not really cover what you have requested, but still is beneficial in terms of grouping like items.