Here’s a new link to check out, Famous Detectives.
This index of detectives, fictional and real, may not be exhaustive, but it does cover a number of well-known sleuths separated by category. Detective categories include: private investigators, amateurs, police, medical examiner, and TV sleuths. Each sleuth is given a bio page of sorts with info about the character. The inclusion of televised sleuths alongside their print fiction counterparts is particularly nifty.
The site has been around since 2010 and likely will continue to grow and expand. Discover a new detective or track down one you might have forgotten at this site, which has been also added to my links below.