There are still corners on the market who would fit well in Detroit's secondary and in their budget (now that Calvin Johnson signed a $132 million 7 year extension freeing up some cap space):
Patterson had a good 2011 season on 538 snaps in Cleveland. He's a good nickel corner which is what the Lions need with Wright out of the picture. If the Lions are comfortable with Houston and Berry as their starters (Schwartz has endorsed Berry several times this season) Patterson would be a solid addition. He has the right mix of age and experience to bolster their secondary.
Allen played behind Jonathan Joseph and Kareem Jackson last season for Houston in a similar role that Berry had with Detroit. He had a decent season which included 4 interceptions, but also gave up an average of 11.7 yards and was burned for 5 touchdowns. He's certainly someone the Lions should look at, but only pick up at the right price.
Hood's age is working against him, but he would provide quality depth as he played well in spot duty for the Rams last season.
Middleton is a restricted free agent and has been tendered at his original round (5th). Jacksonville just spent $5 million on Rashean Mathis, 15.3 over 3 years on Ross and still have Derek Cox, Drew Coleman and Kevin Rutland on the roster. Jacksonville's secondary was plagued with injuries last season, but everyone played surprisingly well which may make Middleton less of a priority. Even if he is tendered at his original round (5th in 2009) it would be a steal for the Lions to sign him.
Who the Lions should stay away from:
Porter will forever be the guy who picked off Peyton Manning to seal the deal on New Orleans' super bowl win. Less memorably, but more importantly, in 2012 he gave up an average of 12.1 yards with a completion percentage of over 70%. This could be because of the Saints' non-existent pass rush all season, but either way Porter will likely command far more than he is worth.
Newman's name has been dropped quite a few times as a stop-gap solution in Detroit's secondary. He is a 33 year old corner whose play dropped like a rock halfway through the 2011 season. His starting experience will likely land him a much larger contract than he is worth, but he isn't Carlos Rogers and he isn't Asante Samuel. Pass.