This is where I'm going to admit that for pure "pump up" purposes, Puffy's cover "Come With Me" works better than Kashmir -- but Kashmir is an INFINITELY better song. Thunderstruck by AC/DC has been listed a few times, and I'll list it again, as it's my number one.

Some unusual ones from me are "Pet"/"Counting Bodies Like Sheep To The Rhythm Of The War Drums" by APC, "Bottom" and "46 and 2" by Tool, "Machinehead" by Bush, Nazereth's "Hair of the Dog" and "Clubbed to Death" by Rob D.

Guns n' Roses - welcome to the jungle!

Wow, Half of my current playlist is mentioned in this thread.

Here's a few off the top of my head..
Wake up - Rage against the Machine
Bombs over Baghdad - Outkast
Cochise - Audioslave
Walk - Pantera
Boom - System of a Down
Push It - Static X
March of the pigs - Nine Inch Nails
Bodies - Drowning Pool

umm.. Rammstein, Rob Zombie, Metallica, Disturbed, have multiple songs that do the trick...

As the lovely ShaniFaye mentioned, it really depends what I am getting pumped up for.

For a particular subset of physical activity,
Steve Vai - For the Love of God and Tender Surrender
Satch - Always with Me, Always with You and Love Thing

For the rest of it:
For some reason Evanescence - Bring Me To Life
Followed closely by:
Bruce Dickinson - The Return of the King
AC/DC - For those about to Rock
Eminem - Lose Yourself
Bush - Machinehead

And I could go on and on and on - one more from the world's best band, though:

Blue Öyster Cult: Pocket

there are too much of 'em, but I'd say
"Guano Apes - Open your eyes"

I've got a few, depending on my mood:

Dreams - Van Halen
Children - Robert Miles
Du Hast - Rammstein
Spybreak - Propellerheads
Seven Nation Army - White Stripes
Jenny Says - Cowboy Mouth

And not necessarily a PUMP UP song, but one that always always puts me in a great mood:

In Your Eyes - Peter Gabriel