Melanie -- Sorry for the delayed response on this one. It was stuck in Tender's spam filter. We've disabled the filter since it was finding too many false positives.

Regarding your question: We don't natively provide support for coupon codes yet but we're gearing up for it. It's an often requested feature and we want to use it, too! We'll get to it as soon as we can.

1) get X months membership free

This is simple. When signing up the customer, override the default initial bill date -- set it to X months in the future. You can even do this without a payment method, then use CG's "Bill Reminder" emails to notify the customer that they must enter payment prior to the end of the "free" period in order to continue using the service.

2) BOGO - buy one membership, get one free

Hmm... No sure what you mean here. I think you mean that if one customer buys a membership, that customer can give a membership away to a friend. In that case, I think you need two "promotions". One that triggers the other. Giving a "free" membership is trivial. You could use another pricing plan which is configured just like a normal membership except it's free. Then, when the "free" membership signs up, assign them the "free" plan.

3) Discount off standard membership price for a period of time

This is a little bit difficult since CG doesn't currently have any mechanism to automatically "remove" a discount after a period of time. The only way to automate this is if the discount only lasts for the current billing period. You can do that with a "periodic" tracked item. For additional flexibility, you would need to set a job to run in the future to remove a discount after your chosen discount period ends.

We have a prototype feature called "tracked item grants" which we intend to use when we introduce native support for promotions.

4) X months free for add-on(s) to membership

You could do this in a round-about way. Use tracked items for the add-ons at the regular price, Then, use a negative tracked item for a recurring discount and schedule another future job to remove the recurring discount after the end of the period.

You could use CG's transaction callback hook to trigger your system to determine if a particular customer's subscription is scheduled to be modified according to your promotion parameters (instead of scheduling a future job).

textpal-1 - There's an option in Settings to turn on/off the payment method requirement on paid plans.

mail -- We're still in spit 'n' polish mode on the latest release (paypal, hosted pages, etc). Also, buttoning up some back-end engineering stuff. We're getting close to new dev mode and will be formulating an updated roadmap soon. Coupons/Promotions/Affiliate mgmt will definitely be on it.