Planets is a fun, interactive program for many-body gravity simulations. The emphasis is on play: it's easy to add in planets, zoom in and out, change the physical constants, save and load configurations, etc. It's designed to be easy enough for a kid to enjoy, but that hardly precludes adults.
SKS (Synchronizing Key Server) is a full-featured replacement for the standard PKS OpenPGP Key Server. It matches all of PKS features and interfaces, and at the same time provides a highly efficient, gossip-based replication algorithm that ensures that the replication is complete.