About a year ago, I found this site, and spent hours pouring over the gorgeous illustrations. At the time, I was interested in adapting the Solomon Kane RPG to a real-world campaign I was calling "Swords Against Satan." These maps would have been perfect for that. I saw today another RPG blog that referenced this site as a great resource, and I completely agree.
Lately, I've been pondering "Warhammer Historical Roleplay." Since the original Warhammer Fantasy Battle and Warhammer 40,000 were tabletop miniature games that were adapted as full roleplaying games, and Warhammer Historical is a tabletop miniatures game, why not adapt that as well? All you would really have to do is remove the magic rules and adapt some careers (of course, I'm using WFRP 2nd Edition as my basis here; I haven't seen 3rd Edition).
Europe in the 15th -17th centuries would obviously be the best base for this, and that's just the period covered by these gorgeous maps.
I may never get around to that little project, but if I do, I will certainly not have to spend any time making maps of cities, and will have some lovely player handouts.
One interesting tidbit: last night I was reading the second volume of the Baroque Cycle, which opens with a description of the city of Algiers (shown above). I was having trouble visualizing the way the breakwater led out to the fortress, so I checked this site. It definitely enhanced my enjoyment of the subsequent pages of adventure in Algiers.
And, since the site has high-res images of all the cities, suitable for printing, I am tempted to even frame a few of these. I hope you enjoy scrolling through these cool maps. I know I did.