- memcached - an in-memory object caching system. For tips on how to integrate it into your database and application layers, you can see the tail-end of my JIT Scalability talk.
- MogileFS - "Distributed (meta) file system. Spray files across cheap disks on your network. Pay less for storage. No proprietary on-disk file formats." Excellent if you happen to be building a site that, for example, receives and stores gigabytes of photo uploads on a regular basis.
- Perlbal - reverse proxy written in Perl. At IMVU, we used its relatively straightforward plugin system to implement a simple long-poll solution for chat.
Thanks, Danga (and everyone else who's contributed to these projects).