I read the Planescape boxset several years ago and I've held an interest in it ever since. I'm currently running a d20 Modern game that is about to make a serious spill into Planescape.
Does the old campaign boxset have the best maps of Sigil or is there another good source I should look at?
Are there any good online sources for maps of the planes or specific planar locations?
You could try http://mimir.net or the Tiefling's exultation on http://www.geocities.com/Area51/Vault/9974/. They don't have a great deal of maps, but have details on planar locations.[/code] Also, neither of those sites have been updated for quite a while.
http://mimir.net/psmush/maps.shtml has some good maps of Sigil, schematic rather than illustrative.
The original box set had a map but it wasn't very descriptive. The best map came from 'In the Cage: A Guide to Sigil' as well as smaller maps of specific sections of the city found in several of the modules such as 'Harbinger House'.