Is there a good place that keeps track of current valid promo codes? I've tried my bestto search but everything I've found is either expired or invalid.
These forums are probably the best place. Whenever new codes are posted, they'll be posted here. I think the S1mple one is still active, I don't think any others are.
I don't think there is a fixed schedule for it. There have been about a half-dozen since they were introduced. My guess is they are still experimenting to figure out how to best use the system, since the last few times were complete disasters from a PR perspective.