The only such products that I'm aware of are Avea's line of Ethernet-connected proximity card readers that, when tagged with a card, send an HTTP request to a web server at an IP address that you specify via DHCP option 72. You build your own web services to respond to these HTTP requests with instructions that the card reader can understand, e.g. with instructions to unlock the door or to deny entry. These support only proximity cards and key fobs; they do not have a magnetic stripe option. There is a model that includes a keypad for doors that require 2-factor authentication.
Of course, this means that you can build and maintain your own database of cards and users... or, even better, you could setup your physical security access control system to query your LDAP or AD servers on the back end.
More information is available on the Avea TCP/IP card reader product page. The WEB08S manual includes detailed information on HTTP requests that these devices generate as well as detailed information on the responses that they can accept and understand.
Athough Avea's implementation is completely open, it is not based on an industry-wide open standard. That's too bad, but I still think it is the most promising option out there. I really like the HTTP approach: any programmer worth her salt knows how to use a database (or LDAP) lookup to respond to an HTTP request. On the other hand, most commercial physical security systems use arcane and outdated communication protocols like Wiegand, have absolutely disgusting access control and authentication systems on the back end, and offer no opportunity for integration with your own systems.