Check out The Island soundtrack by Steve Jablonsky too. IMO it was almost better than the movie itself (ok movie, excellent music).
I don't think anyone mentioned John Powell (Bourne Identity, Bourne Supremacy, and Bourne Ultimatum) or Danny Elfman (he did the Spiderman movies, The Hulk... lots more. I've been meaning to get the Terminator Salvation soundtrack, but haven't yet.)
Marco Beltrami did some interesting ones too, (I, Robot and Hellboy)
And of course there is the soundtrack to 300 (Tyler Bates) and Deathrace (Paul Haslinger).
James Newton Howard is good, but IMO his soundtracks are harder to get into at first since they aren't always as exciting. Don't get me wrong, I recommend his soundtracks, just know that his, and some other's aren't always as fast paced as Danny Elfman
Avatar (James Horner) had a decent soundtrack although not as intense as others mentioned. (same as above goes for this one. Not as constantly fast-paced, but IMO still a good listen depending on your taste).
Now that I've thrown out all those names, here's my list I think you should start with (just suggestions, give or take a few from the above list)...
Hans Zimmer - The Last Samuri
Steve Jablonsky - The Island
John Powell (Bourne Identity, Bourne Supremacy, and Bourne Ultimatum were all pretty good, I have the last two. Pick one
)
Marco Beltrami - I, Robot
Hans Zimmer also did the music to the second two Pirates of the Caribbean movies, although you may or may not care for it. I loved it, but listened to it way to much, so now I don't really like it as much.
Basically just watch action movies and try to pay attention to the music
. Music can make a movie or ruin it (I'm sure you know what I mean if you've ever seen any really low-budget movies). That means the music to good movies is usually pretty good.