For me, it’s sf writer Jack McDevitt, whom I’ve met a couple of times and stay in touch with.
How about you?
Jesus
President Obama.
Although he’s usually too busy to ever write back.
Dude. He writes me all the time. He sent me a nice note right after the SOTU.
Some of my former professors are fairly well-known in their fields, either for their scholarly work or for the government positions they’ve occupied. I’m on good terms with most of them, and have been known to bug them for their views on the news or job recommendations from time to time. 
But other than that, no one. I have very obscure friends.
I guess it’s a toss-up between Bert Kersey and Gary Hart (the California one, not the one who ran for president.) Both first cousins once removed. I don’t e-mail them but I could.
Probably Maria Bello. Not that I have her email address, but if I did, she’d know me immediately and it’d be a nice little reunion. We used to work together and were pretty chummy but drifted apart before she became famous.
Probably Ann Rule. We’ve corresponded about an axe murder that happened 100 years ago just down the road.
No kidding.
I used to know Jim Butcher fairly well in a listserv kind of way, before he got famous. Not anymore, though.
Unfortunately, a lot of them are dead, now. As for the others
There are some famous writers that I could e-mail with the likelihood that they’d know who I was and would very likely respond, but that’s not exactly my definition of “casually”.
Monkey, singer for the classic punk band The Adicts (in fact, I just talked to him yesterday). Certainly not mainstream but if you’re into (or have been into) the punk scene at all you definitely know these guys.
Otherwise, I got nothin’.
I just e-mailed Calgary’s mayor, Naheed Nenshi, yesterday. I knew him back at university and we reconnected during last year’s municipal election and have stayed in touch through Facebook since then.
I used to live with a future NFL running back, but I haven’t talked to him in 15 years so I don’t know that I could casually e-mail him. I know some musicians – for example I’m friendly with a guy in Jars of Clay and could email him easily enough. I’m also friends with a justice on the Sixth Circuit Federal Court of Appeals but I don’t know if that makes her famous.
My (former) PhD advisor is pretty famous among Shakespeare scholars, but not famous to the world at large, so I guess that doesn’t count? (His daughter, OTOH, is a legitimately famous writer, but I’m not sure whether I’ve even met her or what I’d say if I e-mailed her, so she probably doesn’t count either.)
Apart from that, I can’t really think of anybody.
Casually email? Nobody.
But I have engaged in written correspondence with quite a few famous or semi-famous people over the years.
I’ll keep it discreet, but he has been awarded at least three Oscars.
Probably the most famous I could casually e-mail (he’s certainly casually e-mailed me) is Anthony Siegman, Professor Emeritus of Engineering at Stanford, former President of the Optical Society of America, and author of, among other things, the monumental book Lasers.
http://www.uscibooks.com/siegman.htm
He’s probably completely unknown outside the optics community.
No one I would bother casually, but I once got an email from author Samuel R. Delany, who was looking for contact info on a mutual acquaintance. I have no idea how he found me.
John Lee Hancock, Director of The Blind Side, The Alamo, A Perfect World, etc. He sends a lot of behind the scene stuff from premiers, etc.
Mark Hurd, until recently CEO of HP.
Mike Singletary is in a group list of ex-NFLers I hear from, although I’m not going to casually put anything to him, it’s more for reunions, illnesses, etc. Ray Berry is on there too, Van McElroy, Keith Bishop, etc.