As rock musicians get up there in years they tend to get mellower (take Sting, Eric Clapton and Mark Knopfler as examples).
Are there any that have bucked this trend and gone the other way?
Well Tom Waits is as big of a bad-ass as ever, and so was Johnny Cash out to the end.
Not exactly mainstream, but Killing Joke seems to be getting harder as they age (maybe it’s the Viagra).
Carlos Santana still swings a mean axe, and at least as hard as he used to.
Pat Boone tried to reinvent himself as a hard rocker/punk at the tail end of his career.
Chefguy, I’ll see your Santana (I’m not a big fan - I think he noodles around too much), and raise you Jeff Beck - still the gunslinger to beat with power, taste and speed - really tough to beat him and he still rocks hard.
Alice Cooper. Check out Brutal Planet.
If Richard Thompson comes somewhere near you with his band go see him. He’s (and I don’t use this word lightly) awesome!
Dammit Ultrafilter, I thought I was the Alice fan around here! 
Iggy Pop maybe? I think it’s safe to say that he doesn’t appear to be mellowing out.
Slayer.
Neil Young?
Also Ozzy Osbourne (of course his TV dad image and endorsement deals have perhaps hurt his heavy metal credibility). But his music has become heavier over the years, mainly due to his rotation of younger sidemen.
Can’t really think of any, personally. I’d say Santana, but he was already named.
As much as I love old Aerosmith, anything made in the past… oh 8 years really kinda creeps me out. I get the feeling he’s singing to girls young enough to be his granddaughter, irrational or not. Does that count as bucking the trend? lol
ultrafilter, zoogirl, direct your browsers to this, and select Alice from the pulldown menu.
Not as industrial sounding (by a long shot) as Brutal Planet or the followup, Dragontown, but still a whole lotta fun.
Rock harder? The man’s got more energy now than he did in the 70s. 45 minutes into the set, he starts doing jumping jacks. I’m starting to think there’s something to the fact that he’s still doing I’m Eighteen in concert well into his fifties.
I wouldn’t say that he rocks harder, but certainly still rockin’ is David Bowie. I don’t think you could say Bob Dylan has gone smooth also.
And I agree for Johny Cash and Tom Waits.
Motorhead hasn’t slowed down any in over 25 years, and Lemmy will be 58 this year.
Neil Young started out as an acoustic folkie (“Old Man”, etc), and morphed into a grunge rocker, and then to a straight-ahead guitar driven rocker.
The Beatles.
Bob Dylan.
John Hiatt certainly isn’t mellowing any with age. Rather the reverse, I’d say.
I heard some Wayne Kramer (of the MC5) stuff on Little Steven’s Underground Garage radio show and NPR’s Fresh Air that sounded like he was still hanging in there.
I don’t like their current sound much, but King Crimson hasn’t mellowed too much. They even toured with Tool a little while back.
Handsome Dick Manitoba, and the rest of the Dictators rock harder than ever.
I heard some recent stuff from Mark Lindsy with the Chesterfield Kings that rocked fifty times more then anything he ever did with Paul Revere and The Raiders (well, 'cept for “Just Like Me”)
I saw Iggy mentioned, but lets not forget those Ashton boys. As the new Stooges tracks show, they never lost it, and The Cramps’ Lux Interior and Poison Ivy keep getting better with age.
Jon