Actor-Shared Background: Jeffrey Donovan comes from an impoverished family and has twenty years of martial arts experience (including a black belt in karate).
Also, in "Fast Friends," Sam mentions that he once got to the "Michigan State Quarterfinals" with his arm (shot put? football?); Bruce Campbell is originally from Michigan (the Detroit suburb of Royal Oak, to be exact).
Tom Baldwin gets scammed out of a painting by Michael in "Seek and Destroy."
It seems Toby Ziegler snuck out of The West Wing for half an episode.
They seem to be running through Jeffrey Donovan's old The Pretender co-stars. Patrick Bauchau (Sydney) plays Lucien the bombmaker in "Eye for an Eye", and Michael T. Weiss (Jarod) plays the lead bank robber in "Army of One".
Sergeant James Doakes makes a brief appearance as one of Michael's temporary meeting facilitators in Season 3. Mike even injects him with a syringe Dexter-style to make him think he's been poisoned.
Dead Larry is, depending on your age, Vice President Hoynes or Otter. Nerdy LAPD Detective Dutch teamed up with him in the penultimate episode of Season 4.
Harry Dresden comes calling as a terrorist leader bent on capturing Fiona and dragging her back to Ireland. Needless to say, Michael makes sure it doesn't end well for him.
Simon is Cromartie and John Henry! The scene with him slow-motion walking towards an injured Michael in a police uniform with a pistol gripped at his side is a clear Shout-Out. The show seems to have a habit of hiring former Terminators on.
Vaughn is Uriel and Bunny Colvin. A number of others from The Wire also show up. Rawls became a Congressman, Ziggy apparently escaped from jail only to become a Mad Bomber, Dennis "Cutty" Wise and Kima are now in the CIA, and McNulty's ex became a Classy Cat-Burglar.
And Some Other Stuff: In "Lesser Evil." Michael's explosive recipes contain just enough to tantalize MythBusters and brush over the rest to avoid liabilities. Usually. Sometimes it's just to handwave that an explosion is a bit larger than should be expected.