The neat thing about The Visual Dictionary is that the photographs are submitted by users. I'm going to keep an eye out for interesting signage and be a contributer.
I wondered if there was a similar site that had single letters. I found lots of commercial sites that had photographs of alphabets, like Alphabet Photography, but I couldn't find one like the Visual Dictionary. Instead, I discovered a trend called "found alphabets." There are lots of people, it seems, looking for the alphabet in everyday objects. There's even a Found Alphabet flickr group.
I did a google search for Found Alphabet, and here are some really cool images I found. This one from flickr from someone named Chelsea Sektnan: