Miller spent time on loan at Shrewsbury at the end of the season, while the club remain in talks with former Derby forward Porter, whose current deal is due to expire.
Goalkeeper Mark Howard and defenders Bob Harris and Terry Kennedy have been retained, but boss Nigel Clough wants to add more firepower.
Defenders Matt Hill, Sean McGinty, Jasper Johns and Jordan Hodder, midfielders Darryl Westlake and Elliott Whitehouse and striker Jahmal Smith are all looking for new clubs.
Clough told the club's official website: "There have been some tough decisions, but we need to make way for new faces.
"It is difficult, especially when you are dealing with good characters.
"Chris Porter's situation is a little different. We've not taken up his option and because of that he has to go on the list of free transfers but we are still talking with regards to a new contract."
Miller, who joined the club from Crewe in July 2012, missed the second half of the 2012/13 campaign due to a knee injury and his 24 appearances this season came mainly off the bench.
Porter arrived on a free transfer from Derby in the summer of 2011 and following a loan spell at Chesterfield in October went on to score 11 goals in 36 appearances for the Blades.