Best?
It's not uncommon to implement your own, straight from the
specification.
However there is an
open source implementation available.
Also have a look at
this and
this Hessian is a binary protocol that is supported on multiple platforms.
Restlet can be used to serve representations in a variety of formats (and there seem to be a number of C++ implementations of RESTful clients) - though the RESTful community prefers Resource-Orientation over RPC.
It seems more recently
JSON-RPC has also emerged (though it may have stalled).
[ November 08, 2007: Message edited by: Peer Reynders ]