I used to write more about Tigers prospects on Tiger Tales, but not so much now. Given my time limitations and given that so many others are covering Tigers minor leaguers in great detail, I have stuck mostly to statistics and commentary on Tigers news. The top site for Tigers minor league news is TigsTown.com. Mark Anderson has started ranking Tigers prospects starting with numbers 41 through 50. He will post the rest of the list in coming weeks.
Anderson also has a new site (Baseball Prospect Nation) where he'll be covering prospects for all teams. He has done top 15s for the Braves and Orioles so far and the Cubs and White Sox are on deck. One of the best parts of his site so far is his scouting primer. If you are at all interested in how scouts evaluate prospects, this is a must read.
Other sites that regularly cover Tigers prospects are Bless You Boys and Motor City Bengals.
Those two sites do not focus primarily on the minor leagues, but they have about 20 writers between them and delve into every aspect of the Tigers.
There are also a number of national sites covering Tigers prospects. Baseball Instinct recently put up their Tigers top ten prospects list. They very high on shortstop Eugenio Velez and first baseman Aaron Westlake. I will alert you to more Tigers prospect rankings as they become available.