What I do is list the general area of work as well as the skills used, so for example:
- Internal templating language. Details confidential. Skills used: LLVM, HTML parsing.
- Internal search-quality research project. Details confidential. Skills used: clustering, classification, unstructured data extraction, HTML parsing, AppEngine, Django.
- Internal prototyping framework. Details confidential. Skills used: webapp security, cross-domain web requests, rapid prototyping.
That gives anyone looking to employ me an idea of what I've been up to and what skills I bring to the table without giving away the keys to what my previous employer was doing. It may provide some tantalizing hints, but there's pretty much nothing useful there for a competitor to replicate it.