I'll give props to Rise Against like a few others here have. Appeal to Reason really is a sweet album so that usually finds its way on to my playlist. I keep some other various punk like greenday, AFI, Anti-Flag, maybe some screamo works its way in the mix, too.
Mellow park laps I usually change it up and go a bit more mellow. raw delta blues or doo-wop I got a 3-CD set of rockabilly/doowop various artists but for blues I like RL Burnside, Robert Johnson, etc.
Some classics are always in the rotation:
Iron Maiden - Number of the Beast
Metallica - Master of Puppets or ...And Justice For All
Guns n Roses - Appetite for Destruction
Dio - Holy Diver
Lately I've been listening to some more metal like whatever I can find but I really like the scandinavian folk metal like Wintersun, Ensiferum, Finntroll, etc. So many layers of sound there, and some good acoustic bits to change up the pace. I think Ensiferum's 2001 album definitely makes the rotation this winter.
Where I'm riding this year:
Pine Knob - 13
Mt. Holly - 2
Treetops - 2
A-Basin - 2
Mt. Bohemia - 3
Mt. Brighton - 1
Caberfae - 2
Crystal Mountain - 1