Plenty of cameos from Baltimore-area personalities. The Baltimore Sun office in season 5, for example, has several former Sun writers playing themselves in their old jobs.
Many remember JD Williams (Bodie Broadus) as being Wangler from HBO's Oz. At least three cops played medium to minor characters in Oz, as well.
The Greek will seem very familiar to fans of Law & Order. His actor has played several roles on that show, perhaps most notably the Pedophile Priest who molested Mike Logan as a child.
Back in season one, Chris Partlow can be seen in court wearing a police uniform — wait what?
There was another season of The Wire planned which would have dealt with immigration and the large influx of Hispanic immigrants in Baltimore. Since none of the writers spoke Spanish nor knew enough about the immigrants that lived in Baltimore, the season was cut.
During season 3 the idea was floated of spinning off the political subplots into a separate show called The Hall. Instead the political side was folded into subsequent seasons.