hardware loadbalancer + apache 2.2. proxy + mongrels

We (http://pivotalsf.com) have a client/project that uses an f5 load balancer to spread load across three web/app servers running apache 2.2 backed by a mongrel cluster (mysql and solr on separate hosts). This works great. Our only (minor) problems with this have been with the lb's config properly detecting 503s from apache. I highly recommend apache for this.

Hope this helps,

pt.