OmniGraffle is not geared
at doing this, but I personally think it's fairly good at it, depending on how complex your needs are. In fact, Omni just moved to a new office building, and before the move many of us used OmniGraffle to plan how we were going to arrange our new offices. We have also used it to plan out floorplans in the past, such as when we've hired a new employee and needed to experiment with how we might be able to fit them into an existing office.
EDIT: I forgot that setting arbitrary scales is only available in the Pro version of the application. Be aware that if you do this in the Standard edition of OmniGraffle, you'll need to manage the scale yourself.
We have a lot of common furniture - desks and bookshelves etc. which are used in many of the offices. We've measured those pieces and built a simple stencil (which is something that you could easily do as well) so that when planning out a new space we can often just draw out the new space and drag in the furniture we have for our offices.
But I encourage you to try out OmniGraffle for yourself before you decide whether or not you want to purchase it. By default it will allow you to create simple diagrams without a license, and you can also get a daily free trial license from our online store in order to unlock all features of the application for at least a day (and you can come back for another license tomorrow if you need more time). This will also help you determine if you will be satisfied with the Standard edition, or if you want the Pro edition.
You can download OmniGraffle from this page on our web site:
Note that there are separate downloads for the Standard vs Pro versions. If you download one and then request a trial license from our online store, make sure that you request the appropriate license - a Pro license will not work in the Standard version, and so forth.
Also, there is a link on that page to the public beta of OmniGraffle 5, the next major update to OmniGraffle. I suggest testing the beta if you are running Mac OS X 10.5 (which OmniGraffle 5 requires) - while version 5 still needs some polishing before it is ready for release, it has a lot of improvements over the current version of OmniGraffle.
Note that the price of OmniGraffle 5 is going to be higher than the current price of OmniGraffle 4. However, if you purchase now you will get a free upgrade to version 5 when it is released.