i think someone has mentioned this elsewhere already but a wallet service that had subscription functionality would be good.
Explain in details?
i wish i could find the thread where someone else explained it but a good comparison is alertpay.
i think paypal also caters for subscriptions.
let's say you have a 'membership' site that you want to charge 3 BTC per month for access.
a user subscribes to your site via a wallet service that caters for this. 3 btc is charged and the user is given access. (this part is no different from current set ups that are available right now... it's a callback to your server where you authenticate the payment).
then every 30 days, an extra 3 btc is automatically sent from the user to the merchant for continued access.this is initiated by the wallet service (cron job?) where the user should keep at least 3 btc available for his next month's subscription payment.
if the user doesn't have 3 btc available in their wallet for that month's payment, then their subscription is cancelled and they are unsubscribed from the site. (or optionally, they are sent a warning email asking them to top-up their account so the payment can be re-tried in 24 hours).
the merchant who runs the membership site could have options to set up all kinds of subscriptions depending on what the subscription is for. eg, they could set it up so that it cost 0.05 btc for a 3-day trial, and then 1 btc per month after that.
anyone implementing such a feature should definitely look into the way alertpay handles this.
with other options like alertpay, shorter subscription periods (i.e. daily) are impractical because of the high fees... you can't do $1 / day subscription because it'd cost the member $2.50 / day after all the fees, and the merchant would still only get $1 out of that. but with bitcoin it becomes feasible to have much shorter rollover periods.
...as a side note, there are sooooo many websites that i can imagine making use of such a system.
eg. bitcoin monitor is free the way it is now, but 0.1 btc per month for more detailed graphs.
mtgox is 0.65% commissions, but pay 5 btc per month to have unlimited 'free' trading for that month.
various btc websites that now contain ads, you could pay 0.05 btc per month to use an ad-free version.