When I first discovered the Internet, the first thing I did was sign up for Dorothyl, a mystery-lovers news group named for Dorothy L. Sayers. Not only do mystery readers participate in dorothyl, but also quite a few of today's best mystery writers.
DorothyL is a large mailing list about all aspects of mysteries. You can get information about DorothyL by sending e-mail to firstname.lastname@example.org with the message "get dorothyl welcome f=mail". You subscribe by sending the following message to the same address "subscribe Dorothyl (first name) (second name)" (with your name and without the brackets). You can also access a list of home pages created by members of DorothyL, an introduction to DorothyL, the DorothyL Archival Database and Miss Lemon with DorothyL material. The post from DorothyL can also be accessed by the bit.listserv.dorothyl newsgroup
I've also been signed up to The Hounds of The Internet for several years. As you might guess, this group discusses the various books dealing with Sherlock Holmes. Information about the Hounds and everything else Sherlockian may be found at the Sherlockian Holmepage.
There are also pages for other mystery characters. One is:
I've been a big fan of PBS's Mystery series for years and, now that I've set up a satellite dish, also enjoy the mystery programming of A & E (although things currently seem to be going downhill).
There are a number of other sites, dedicated to mysteries in general or those run by mystery bookstores. The Mysterious Homepage has so many good things that it makes an excellent starting place for finding just about anything.
You can also find good things at the sites listed below: