When you go service oriented, either REST or SOAP or whatever other hybrid, you will need sooner or later a management layer on top of you services including authentication, throttling, documentation, provisioning, caching, etc. Here's a collection of API management tools which cover either the whole spectrum of this functionality or a single piece, both commercial and open source.
Commercial
Open source
- apiGrove
- WSO2 API Manager
- api axle
- REPOSE
- varnish-apikey
- Google API Explorer source (documentation/discovery only)
Other resources
Enjoy and feel free to add in the comments any others that you know of!