ESPN—It looks good, it's simple to read and you can view it by "all NFL injuries" or specific teams.
CBS Sports—A very cool feature on this injury report list is the "Fantasy News" which often includes reports from local newspapers on the status of a player's injury.
NFL—Well, it is the sport's official website. Plus, it's easy to read and gives some details on a player's "Practice Status" during the week.
ProFantasySports—These guys do a great job of staying on top of the latest injuries. And they include a nice feature called "Projected Return" to give you a likely return week.
Rotoworld—For some odd reason they alphabetized it by the team name and not the city or state. Nevertheless, we still like the "Latest News" feature on it.
Tip: If you're still unsure, search for a particular player's name in Google News. Many times a local newspaper will have the scoop on a player's status.