Absolute Write has a great forum for mystery writers.
The Writers Write website offers links to mystery research tools: "finding crime resources;" "crime and police procedure;" "forensics;" "government sites;" "types of crime;" "equipment, weapons, and poisons;" etc.
Mystery Readers International has links to mystery-related publications and bookstores, and has a journal you can access online with articles on topics like "academic mysteries," "ethnic mysteries," etc.
The Bloodstained Bookshelf publishes lists of recent and upcoming mystery releases.
There are several Yahoo Groups you can join for more contact with other writers, as well as research information and other support: 4 Mystery Addicts, Murder Must Advertise, Crime Scene Writer, Wicked Company, Weapons Info .
The most popular listserv focused on the mystery genre is Dorothy L. (as in Dorothy L. Sayers.) To subscribe, send an e-mail to email@example.com with the message "subscribe Dorothyl (first name) (second name)" (with your name and without the parentheses).
You can check out a couple of newsgroups related to mysteries, too. http://groups.google.com/group/rec.arts.mystery/topics?lnk=srg&hl=en and http://groups.google.com/group/alt.pulp/topics?lnk=srg&hl=en . Once you get used to the layout of Usenet groups, you can find interesting articles, topics, and information.
Facebook also has groups devoted to mysteries. Search for ""Sherlock who?" The Ellery Queen Appreciation Society!", "The Golden Age of Detective Fiction," "MWA (Mystery Writers of America)," " Alfred Hitchcock's Mystery Magazine," and (for all writers) "Poets & Writers."
The Mystery Writer's forum http://www.zott.com/MysForum/ ( http://mwf.ravensbeak.com/forum/index.php?topic=664.0) has useful links on everything from forensics (including crime-scene cleanup, drugs and poisons, and crime scene investigation) to forgery and hacking.