Mike’s Amazing World of Comics.
A web site where you can use a control panel thingy to enter your chosen month and year and get thumbnail images of all comics that made their first appearance on the newstands of America that month. Clicking on an image gives a larger image, publishing and creator details, plus the opportunity to view earlier and later issues of the title.
http://www.dcindexes.com/timemachine/index.php
Health Warning. This site has the capacity to consume great gobbets of time without warning or awareness. Use with discretion.