I highly recommend http://spreedly.com. Has a great API and recurring billing is built-in so you don't need to pay extra for it on PayPal or Authorize.
Spreedly has a per-transaction percentage charge on top of all the other per-transaction fees you pay to the gateway/bank/etc. Not a great option long-term. http://cheddargetter.com and http://chargify.com are much better options. CheddarGetter is a very robust service with usage tracking, branded email communication and microbilling features. Chargify includes a drop-in generic interface like Spreedly.