hCraft has a very powerful block physics system in place.
Initially, all worlds share a single block physics thread spawned by the server at startup time. At any time, the user may choose to allocate a set of threads for any world - from one thread up to 24 (although I doubt anyone would need that many, ...unless you have a 12-core system).
hCraft supports various custom blocks - regular blocks that have a special behaviour associated with them. Internally, they are usually assigned a custom ID (starting at 2000), and are converted transparently to the right type of blocks when sent to the player. Currently implemented custom blocks include: activewater (the type of water that existed in classic Minecraft), Langton's ant, sharks, fireworks, etc...
Custom block behaviour can be also attached to regular blocks, using the /bt (block type) and /bp (block physics) commands. It is possible to convert any selected blocks into doors (that is, classic MCSharp/MCZall doors) just by selecting a bunch of blocks and typing /bt door.
World physics can be stopped, paused, and resumed at any time using the /physics command.