Intended for authors of EMERGENCY! or ER fanfic, RESOURCES FOR MEDICAL FANFICTION WRITING offers plenty of useful links.
You might also try RESEARCH AND GRAMMAR LINKS, which has an extensive list of dictionaries and thesauri, tips for adult and slash writers, and links to sites on medical issues, law enforcement, and medieval/New Age/Wiccan sorts of information.
BUFFY THE VAMPIRE SLAYER fans, Giles doesn't go around calling everyone he dislikes a "pillock." Don't limit his vocabulary; learn some British slang at sites like A DICTIONARY OF SLANG.
For police jargon, try POLICE JARGON: HOW TO TALK LIKE A COP.
Go here for a dictionary of hard-boiled slang ala Bogart detective movies.
Review helpful hints on HOW TO WRITE ALMOST READABLE FAN-FICTION.
Read stories about shows like these.