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?
For general solutions that don't rely on SPARQL 1.1 features there is Olaf Hartig's SQUIN or the Semantic Web Client Library from Chris Bizer et al which may be useful to you.
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.
Allegrograph has a federated triple store option which can create a union of triples stores. Thus, giving you federated queries sans SERVICE keyword. See Allegrograph 3.3 Federation Tutorial
See also: http://answers.semanticweb.com/questions/1053/distributed-triple-stores
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.