As I scour the web for information about uncatalogued cards, one of the main areas of interest is always Major League team-issued cards and photos. I've posted a bit about some of these in the past:
1987 Cleveland Indians Photocard
Team-Issued Stadium Give-Away Sets in Japan
1994 Chicago Cubs Old Style team issue photo card
1992 or 1993 New York Yankees postcard set (update)
They can range anywhere from fully featured baseball cards to over-sized photos to postcards to simple black & white cards with blank backs. They have been around for nearly as long as the hobby, but they definitely don't get the love or attention they deserve. These days, the most common way to acquire them would be as part of a "fan pack" from a team.
3 hours ago