Nothing says spring like the peepers.
A couple of years ago, I published some (decidedly mediocre) photographs of the spring peepers calling just down the road.
Although I plan to get better pictures eventually, I haven't yet returned to to take more. Instead, this year, I got a little recording gadget for my iPod and captured aural rather than visual representations.
I can't think of any sound that says “spring” to me in a more visceral way. It means winter is over (or almost, they start pretty early), it means hot, sunny days and warm evenings, it means flowers and grass and leaves and bike rides and, well, you get the idea.
At a distance, the calls are soft, splended, and beautiful. Up close, the calls are still beautiful, but they can be just a little overwhelming.
These samples are from a small vernal pool on a cool evening well past the peak of the season. A full chorus in a large swamp at the height of the season is a thing to behold. Sometimes with earplugs.