In Riastrad
Anything by Daft Punk, the faster the better.
Also:
"Aces High" by Iron Maiden
"TNT" by ACDC
"King of the Fairies" and "Born to the Fight" by Waylander
"I Am The Doctor" (The Eleventh Doctor's theme song)
"Get Over It" by the Eagles.
Yeah, it's an extensive list, but keep in mind I listen to music at least twelve hours a day.
My name is Cu Chulainn. Beside the raging sea I am left to moan. Sorrow I am, for I brought down my only son.
Lazy?
"Indestructible" by Disturbed once improved my score.
Other than that I tend to find music distracting during most tasks (but I do have a jogging playlist on my iPod).
jackedup85
It really depends on the genre for me.
For action or FPS games,I'll normally listen to anything that's heavy, which keeps me pumped to cap as many asses as I can. Chances are, my tracklist will include "Mein Herz Brennt" by Rammstein and "You're Going Down" by Sick Puppies.
For Stealth games, I like listening to Jesper Kyd's scores for the Hitman series or Amon Tobin's work for Splinter Cell: Chaos Theory.
As for Sports games, I'll most likely listen to random alternative music if I'm playing FIFA and rap music from Tech N9ne, Mos Def, and Tyler The Creator if I'm playing NBA 2 K.
edited 22nd Jun '12 2:01:54 PM by jackedup85
Face Time
For MegaMek, I like to put on Command And Conquer music from Tiberian Dawn and Tiberian Sun.
"Hipsters: the most dangerous gang in the US." - Pacific Mackerel
Most anything by Sabaton, preferably something era-appropriate.
Disturbed's "Indestructible" however reeks a little too much of hubris, even if many player characters are, in fact, indestructible masters of war.
edited 22nd Jun '12 2:41:17 PM by Balmung
Thinking off the top of my head, any good background theme in fighting games does the trick.
the Prodigal
Cornered from the Ace Attorney series in any form works well.
Artificial Intelligence is no match for Natural Stupidity.
Bunny
It goes with everything! 
Perpetual Noob
Wiping All Out
Most recently, this piece
manages to get me pumped up for the next battle.
Ehh, I usually pick genres for stuff, but generally if it's something I need to work hard at to win, I'll put on either some hard-hitting indie rap or punk rock, otherwise I'll just put on some softer indie rock/rap/jazz/chillout. I think I've listened to Jazz Nation Radio outside of GTAIV more than I have during actual gameplay now.
Je Suis "Aware"