I wonder what the current options for federated SPARQL querying are. I'm aware of ARQ's features but what other frameworks or libraries are out there? Does anyone have concrete experiences concerning federated SPARQL queries in a real-world setup?
In terms of pure SPARQL solutions my own framework which I develop (dotNetRDF) also supports the
SERVICE keyword in it's SPARQL engine just like ARQ does in Jena.
As far as concrete real world experience goes I can't help you there I'm afraid.
Indeed OpenLink Virtuoso 6 does support federated SPARQL querying from version 6.1.1 onwards...
Cited from this press release
SERVICE parameter passing for basic Federated SPARQL (SPARQL-FED)
For a server side implementation of federated SPARQL, Virtuoso 6.x claims to support the SERVICE keyword.
EDIT: An emerging product from Revelytix called Spinner will soon be one more option.
I wonder if we should consider DARQ (http://darq.sourceforge.net/) as part of the available solution? it is indeed not under active maintenance.