Here's the reason I'm looking into Arduino:
I have a 1983 SeaRay boat that has TWO V8 engines that don't have any electronics other than electronic ignition modules. Some people will think I'm crazy, but I'd like to build a form of simple ECU so that I can build a form of ODBII for the engines. Other than the ability to output to an LCD panel to make my old analog gauges a thing of the past, there are several other things that could be done for a boat engine like an engine synchronizer, auto start sequence, etc.
I'm sure that I could even connect the unit(s) to the internet and check on my boat from my house, and even warm up the engines once a week.
I have seen a few people tinker with single cylinder engines, but I haven't seen any information on something to this level. I'm sure that an Arduino would be a much cheaper alternative to an expensive replacement of an ECU on a newer car as well.