I just did a presentation at nyc.rb on integrating multiple rails apps:

When One App Is Not Enough

Here are some links to the projects I mentioned ( let me know if I am missing any):

multi_app_routing (http://github.com/jakehow/mutli_app_routing)
braid (http://github.com/evilchelu/braid)
supervisord (http://supervisord.org)
monit (http://www.tildeslash.com/monit/)
god (http://github.com/mojombo/god)
bj (http://github.com/atmos/bj)
starling (http://github.com/starling/starling)
beanstalkd (http://xph.us/software/beanstalkd/)
auto_cron (http://github.com/fhwang/auto_cron)
sinatra (http://github.com/bmizerany/sinatra)
halcyon (http://github.com/mtodd/halcyon)
merb (http://github.com/wycats/merb-core)