I'm, of course, most interested in baseball cards, but the site has cards from all sports and a number of non-sports card sets as well.
You can also use it to catalog your own collection but I don't plan to do that. I have a well ordered database of my own design I use and don't see much point in duplicating it on-line.
The site has a number of interesting lists which I'm going to share on occasion. One of them is "The most common cards in member's collections". This is the baseball card list, which is heavily loaded with late 1980s Topps cards.
1987 Topps #648 Barry Larkin
1981 Topps #675 - Atlanta Braves Team Card
1987 Topps #735 Rickey Henderson
1988 Topps #102 Barry Larkin
1987 Topps #773 Robin Yount
1987 Topps #300 Reggie Jackson
1989 Topps #515 Barry Larkin
1987 Topps #386 Mark McGwire
1987 Topps #80 Wally Joyner
1989 Topps #784 Steve Avery
The inclusion of these Topps sets in this list may be more a function of the prodigious number of sets produced by Topps rather than their being particularly popular with collectors. Having said that, 1987 is one of my favorite sets.
To contrast the baseball card list with the basketball card list, the #1 basketball card is the 1990-91 Hoops #236 Jeff Hornacek with 198 members. All of the cards on the basketball list are from the same set.