I travel to all my games. I carry a legal case (9" x 14" x10"), my mini's case, and a battlemat. It can be tough to navigate doorways and corners!
When I DM, I carry a DMG, PHB, MM, occasionally one of the other books (PsiHB, MM2, splatbook) if its something I will need and the players don't have it. In one group, we play at the house of someone who has pretty much all the books I have and more besides; in my other group we play about 30% of the time at an apartment with all the books.
The players generally bring what books apply to their PC. I don't bring those extra books unless I will also need to check that info regularly.
I concur with Henry about limiting sourcebooks - there's so much out there, it's easy to become swamped in sourcebooks. I try to write out any bits that are taken from otherwise rarely used books in order to limit the amount of stuff I need to carry.