I work at an ad agency so it's an eclectic mix. People working in the creative portions can get away with wearing pretty much anything they like and not get/feel hassled. A lot of the older people and execs stll wear shirt and tie. Most of the guys I work directly with (web developers, programmers, IT admin) wear the prototypical business casual for guys:
-Dockers (khaki & navy blues)
-Polos or casual long sleeve button-ups on most days.
Lately, I've been wearing more and more of those slightly ribbed, solid-colored, v-neck t-shirts (Alfani is the brand I usually get I think.) They look good with my usual casual pants, but also look surprsingly cool with dressier pants (and even look cool with shorts.) I just get sick of wearing collared shirts all the time and it makes for a hint of variety. Plus it's summer and it's the South so it's a lot cooler temperature wise.
Suits are a must for new business meetings, but once we know a client it's typically back to business casual.