My last job was with a company called Braintree, who make world class communication gateways --- mainly for the eftpos and financial industry. So I could always answer "I help design boxes to connect old EFTPOS machines to the bank using new technologies". It glosses over a lot, is largely incomplete, but remains something most people could comprehend. The worst job I've had to try and explain was my first. "I am writing the user-interface to an Electron Paramagnetic Resonance simulation"? I've seen geeks eyes glaze over on that one :).
So far for my current job I've had to revert to the classic "I'm a programmer in an IT company", which is sufficiently devoid of information to be mutually unsatisfying. So what am I currently working on? I am employed by Tucana Technologies to work on an open-source RDF database Kowari. Kowari is used by Tucana as the base on which we build our enterprise rdf-datastore TKS. OTOH, if having difficuty explaining my job is the price to pay for having this much fun I suppose it's worth it. Tucana is a great company to work for, and Kowari is really fun engineering. Still, if anyone can think of a one or two sentence description that isn't quite as insipid I would appreciate it.