Some people have a tough time doing certain things (working out, writing, sleeping,) in complete silence. While I prefer to write in silence, I find rain sounds (especially with thunder) knock me out better than any sleep aid, and I tend to sew better if I'm listening to the some ambient bird or garden sounds.
If you're looking for the same, here are some links for you:
Ambient-Mixer.com not only has a ton of ambient soundtracks for you to listen to, it also allows you to mix your own.
Calm Sound offers a nice variety of nature sounds along with accompanying videos; the relaxing mix is my favorite.
For when you can't get outdoors, here are 11 hours of sounds from an English garden.
Fresh daily rain sounds site Rainy Mood claims rain makes everything better. I think they're absolutely right.
Simply Noise offers three types of ambient sound -- white, pink and brown (I love the brown, actually.) They also explain why they labeled the sounds with colors.
Soundrown.com offers ten ambient sounds for you to listen to (and you can combine them by enabling more than one.)
This Youtube fireplace video allows you to watch as well as listen to the flames.
Waterfalls are natural white-noise generators; here's a Youttube video with fourteen minutes of waterfall sounds.