Ask and ye shall receive.
The best one that I know of is www.sportspot.net
I'm MtGrizzly on that board (Ha!) and it's the best run sports message board that I've ever seen. You have to sign a contract when you sign up, before you can post. It details the things that will get you kicked off and if you violate, you get booted. No swearing, no adult material, no personal attacks. There are some amazing folks that post there - people that know more about baseball than I ever will, that's for sure. I like the Mariner forum and Minors forums on the Spot. Enter at your own risk, though. It will suck time from your life.
For good editorials on players and such, I like the DrDetecto site http://detectovision.com/
The guy is a bit over the top in his optomism toward players but it's always a good read. He'd got two "POTD's" (short for player of the day) up on Fruto right now. One breaks down his pitching mechanics and one details why the author thinks he should be starting. Good, good stuff. I like to check on his site once a day during the season.
Jason Churchill just started a blog called "prospect insider" that is looking really good.
http://prospectinsider.wordpress.com/
The boys at Lookout Landing do an oustanding job if you are into the stats of baseball. www.lookoutlanding.com
They graph more obscure stats than I even imagined. The guy that runs it is a former minor league pitcher, so when he talks about pitchers everyone listens.
I guess I would be remiss if I didn't mention the oldest "unofficial" M's site www.ussmariner.com
They have some good content on there sometimes but it's run by some very bitter people. They seem to think that their word is the truth and if anyone questions the moderators on anything, they go on the offensive. They even delete posts from people that prove the wrong or call them on their BS. It's weird to me, since it's just some dudes talking about the Mariners but I guess it's their site and they can do whatever the hell they want with it. I rarely go there anymore - too damn negative.
LookoutLanding has by far the most entertaining game threads out there. I like reading them after an ugly loss - there are some characters that post there. The interface kind of stinks, though, so I'm usually on the Spot for their "kindler, gentler" game thread.
Enjoy.