As quite a few have asked via the comment form: it's a nginx + passenger + ruby 1.9.2 stack, with solr for indexing, mongodb for data back-end, rails fragment caching in redis, and a warmup script to fill the caches.
Well I'm more used to Redis, as I've used more than a couple of times with Resque (job processing in Ruby).
I started the crawler behind HackerBooks with Resque and Redis, but ended up saving some RAM by going back to a simple daemon. I reused Redis to provide the caching.