www.runtheplanet.com is a site with descriptions of over 4200 routes in over 3100 cities all over the world. It's been running since 1996 and is still going strong. I find myself quite often referring people to this site when they are looking for places to run on a trip somewhere, especially to other countries. I've added a couple of descriptions myself. I encourage you to take a look and while you're there, add a few of your favorite routes.
There are other forums, newsgroups, and bulletin boards where you could find more information about running in specific cities by posting a question or by searching the archived messages. However, what I like about runtheplanet.com is that I can simply type in the city name and it will take me to a page with descriptions of all the routes for that city. The difference is that it is solely focused on running routes unlike other media where there is a lot more information about a lot more topics.