The 32-year-old has made 486 appearances for the club - the second highest total in their history - but has not featured since January 2007 and is one of 12 players put on the transfer list by Darren Ferguson.
He said: "It's time to go now because I want to play football again. The loan spell I had at Bury last season was so enjoyable it made me realise what I had been missing.
"I can't complain about what's happened to me at Posh because
they've been so successful in the last two seasons. I certainly don't hold any grudges against anyone at the club because tough decisions have to be made.
"Posh gave me my break in football and I've enjoyed every single minute of my time here. I've enjoyed a special relationship with the fans and that's something that will stay with me forever.
Posh will always be in my heart and I hope they carry on being successful. What they have achieved in the last two years has been unbelievable.
"I have spoken with Luton and I have also been in touch with Bury, but I'm not in any rush to make a decision because it has to be the right one. I would have no problem dropping down to the Blue Square Premier League as there is no chance Luton would be there for long."