Spikes' deal is for three years, Clary's is for four years and McMichael got a one-year deal.
Safety Bob Sanders previously agreed to a one-year contract.
The players' contracts are expected to be signed on Friday.
"All four players are veterans and have performed at an extremely high level," coach Norv Turner said. "Obviously Jeromey has put a lot of work in with us to get to where he is and we're excited to have him back. The continuity of keeping the offensive line together is big.
"I think defensively you want to be as strong as you can be down the middle and with Spikes and then Sanders, you've got two strong personalities that bring a real aggressive presence to the defense."
A hard-hitting, veteran entering his 14th season, Spikes got a call from general manager A.J. Smith on Tuesday morning, when teams were allowed to negotiate with free agents.
"He is an outstanding player. Takeo will bring his physical, aggressive style to our defense," Smith said. "I love his competitiveness and instincts. I believe he will be an immediate contributor."
Clary has started 42 regular season games for San Diego the past three years.
"I'm extremely happy that I get to stay here in San Diego with the guys I've worked so hard with the last five years and continue to build and get better and work our way to a Super Bowl," Clary said.
McMichael, a 10-year veteran, caught 20 passes for 221 yards and two touchdowns last season, his first with San Diego.
Copyright 2011 by The Associated Press