What the heck is a OPTIONS method in a IIS 7.5 web Log?
Check out section 9.2 of this W3 Spec:
The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.
This blog post is also worth a read.