As you probably have discovered, the net is littered with interesting ambient noise, productivity, and curio shops of aural horrors. While sites like focus@will and Coffitivity purport to tweak attention productivity, others bring novelty and nerd ambiance into our physical and/or head spaces. Check Star Trek TNG Ambient Engine Noise (Idling for 24 hrs) on YouTube.
As a sonic nerd, music producer, and DJ, who has forever been fascinated with found sound and field recording, my favorite site, by far is myNoise – Custom Background Noise Machines. Dr. Ir. Stéphane Pigeon is the founder and designer of this plethora of various types of background sounds, full sonic scenes, and noise (water, wind, waves, monks, monkeys, etc). All scenes/sounds are recorded and presented in multi-track format with an animation mode that guarantees no repeat performances, for randomization fetishists.
The jungle and nature scenes might just pull you out of your seasonal affective disorder.