Good thread, how about moving these links to the links section or making this a sticky please. Tim Stannard recently suggested I take a look at Atmosphere deluxe. It generates soundscapes and has a multitude of them inbuilt and with the capacity to create your own. I was so impressed with it I bought the full version which was £30.
What I think is really good about it is that you can utilise your own sound files (and thus all of the ones mentioned above), include MP3 and CD tracks etc.
It works on the principle of a background noise, e.g. waves rolling in on a shingle beach. This is then supplemented by whatever you choos to lay on top. So, say you want seagulls. Tick the box for seagulls and set a randomness factor and volume. You can also fire those sounds of manually whenever you want. You can then record your scene to a Wave/MP3 file for inclusion in your project. Its simple and I have to say very very good.
Atmosphere Ambient Soundscape Generator - Create custom wellness / ambient natural sound MP3 and CD audio tracks for relaxation or royalty free distribution