The other thing that occurs to me is cheap DIY door soundproofing. We hang blackout curtains on my son's windows so that he can nap, but we made the discovery that it also really blocks incoming sound from outside. Not completely, but it really helps a surprising bit.
I get very easily distracted by some kinds of sounds (I'd rather work in a windowless closet somewhere than a cubicle, and I'm a proud member of Team "All Ticking Clocks Must Die"), and I've found simple things like that surprisingly effective at home, too. One particularly cold winter, I hung a thick, heavy sleeping bag over the window my computer's near, over the blinds and regular curtains, just trying to keep it a little warmer without turning the heat up (very successful with that), and while it obviously didn't block everything out, it made a noticeable difference. I was very, very sad that spring when it finally got so warm that I was forced to take it down and open the window, and the noise level immediately jumped at least a good 30 dB. It's not like I can wear my Etymotics all
day, or my ears start to feel funny. Now every year I impatiently wait for it to get cool enough that I can cover the windows again. Heh.