TMZ has obtained a total of 41 letters written to the judge in Peck's criminal case, and while there were scores of Hollywood types lending their voices in support of the convicted child molester -- all defending his character and asking for leniency -- these 3 are the biggest names.

Marsden's letter is the longest, and he details his extensive personal history and friendship with Peck, saying he'd known him for 14 years (mind you, he wrote this in '04, so they go way back apparently) and generally vouching for his character.

JM writes, "My name is James Marsden and i am very proud to say that i've been a close friend of Brian Peck's for about 14 years now. I am from Oklahoma originally and i met Brian through an actor friend ... Brian believed in my talent as an actor and encouraged my decision to move to Los Angeles and pursue this dream i had."

Marsden goes on to say he and Brian remained close over the years, saying BP mentored and coached him -- all free of charge -- and that they formed a very close bond, with Brian serving as a role model and instilling upstanding character traits.

Marsden says based on what he knows about Brian, he "couldn't breathe" when the charges came down. He says he had lived at his house for months and had shared hotel rooms with Brian -- and he says he never felt uncomfortable or compromised being around him.

He finishes by writing, "Brian is a good person with a very healthy fear and respect for the law and, i assure you, what Brian has been through in the last year is the suffering of a hundred men. I don't want to victimize Brian, nor would he ever wish that, but i do feel compelled to shed light on the fact that (he) has learned his lesson."

Taran -- who worked on the "The Amanda Show" and who also knew Brian very well -- was equally praising in his own letter of support to the judge presiding over the case, calling him his "co-worker, my friend" and saying he's not someone who belongs in jail.

TK writes, "When I found out about Brian being arrested I was shocked. Brian is the last person I would expect to be charged of criminal activities. My first thought was 'this is a mistake.' I know many of our mutual friends had similar thoughts. It is too out of character."

Finally, there's the late Thicke ... whose letter was much shorter but just as supportive of Brian, commenting on their days together on 'Growing Pains,' a period in which Alan says Brian was a "highly professional and nurturing mentor of the young people on our television set." Thicke says BP was never inappropriate.

There are dozens of other letters by people who went to bat for Brian -- including "Boy Meets World" stars Will Friedle and Rider Strong, "Growing Pains" star Joanna Kerns and 'Suite Life' directors Rich and Beth Correll ... whose show Brian worked on briefly, post-conviction.

You can see their letters here in full too ... but there are even more, filed by actors like Ron Melendez, James Marshall, Kimmy Robertson and other showbiz players.

As we reported ... Drake is set to detail exactly what he says Brian did to him before he got his own show on Nickelodeon on the new documentary 'Quiet on Set.' "Quiet on Set' premieres across 2 nights on ID, starting at 9 PM on March 17 and 18.