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.

That's a great stack. Any reason you went with Redis for the caching instead of memcached?

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.

So well: I mostly used what I was used too.

Definitely makes sense if you're already using Redis for something else.