In the past, when I've lived within 2-3 miles of work, I've walked and I loved it! I started doing it one spring when it was just horribly dark and rainy right into May and June, and I could feel myself getting depressed from not spending time outside. Come to find out, the walking commute was very pleasant, even when it was a little rainy, it only took a few minutes longer than my bus commute, and it was way more predictable (no worries about traffic and late buses). Walking isn't 100% free, though - I wore through shoes like nobody's business!
Now I live 3-4 miles from work, and walking seems like it would take too long (I currently bike), but maybe I should add it back into the mix - it would probably take me a little under an hour, and it might be nice to mix it up a couple of times a week.