Sorry, I am stealing someones idea but I thought it was great and haven't seen one for Northampton yet, so credit to whoever thought of the idea!
David Buchanan - been at Northampton for a number of years now, not sure if he is still captain but was for a long time.
Leon Barnett - played for Northampton since being released by us.
Gregg Wylde - we had him on loan from Bolton a number of years back, was on loan at Northampton in 16/17 season.
Michael Smith - The big underated striker we had last season played on loan at Northampton in the 16/17 season.
Ryan Cresswell - centre back we signed following his release from Sheffield United in 2008, played for Northampton between 2014-2016.
Nicky Adams - needs no introduction to us, played the 2015/2016 season with the Cobblers.
Shwan Jalal - Actually signed for Northampton following his release by us, made 4 appearances before being released and snapped up by Macclesfield.
Tom Newey - Played for us in the 2009/10 season. Joined Cobblers on loan 2014/15 season before signing permanently in 2015. Injury prone time there and hung up his boots following his release from Cobblers.
Clive Platt - He will come up in most played for both topics I think! We signed him from Northampton following his release.
Hallam Hope - Made 3 appearances on loan from Everton scoring 1 goal before signing on loan for us the same season.
Joe Widdowson - Another player we signed following his release from Cobblers. Played 64 games in 2 years for NTFC but only made 1 league appearance for us.
Nathan Cameron - Made 3 appearances for the Cobblers on loan before we signed him on a permanent.
Lewis Young - Made 30 appearances for Cobblers in the 2011/12 season. Made 4 for us in the 2014/15 season.
Bas Savage - The man, the legend, the myth. We had him on loan in 2005, amazingly was still playing pro football when Northampton signed him in 2011!
Marlon Jackson - Played on loan for NTFC in 2011, ripped up trees for us in the 2013/14 season scoring 1 goal in 8 games.
Shaun Harrad - Another player we signed from Northampton. Quite a trend here.
Danny Swailes - cracking defender for us, played 3 games for the Cobblers on loan in 2011 from MK dons.
Mark Hughes - Centre back, made 93 appearances for NT between 2007/09. Played for us 2011-2013.
Giles Coke - famously played on loan for us 2011/12. Played 2 seasons at NT between 2007-2009.
Tom Youngs - hard working striker who barely scored played 26 games for Northampton between 2003-2005 without scoring. He played for us between 2005-2007 scoring 7 goals in 49 appearances, mainly off the bench.
Steve Haslam - Mr no knees. Played only 3 games for Cobblers in 2004. Had 3 years at Halifax before joining us in 2007 for 2 years.
Paul Rachubka - He made a career out of being a back up keeper. Played 10 games for Northampton on loan from Charlton in 2004. Signed as a back up for us in the 2016/2017 season, playing 1 league game. Went over to India to play for a season following his release from us.
That is all I can find from the turn of the century. Feel free to add more!