I started freelancing as a personal assistant when I lived in San Francisco – now that I’ve moved to Pittsburgh, I’ve kept a lot of my SF clients, and I work for them long-distance (which means working from home). I basically do whatever people need me to do, which frequently means research of some kind, online office/file organization, or building websites (I don’t do major web design, but I’m pretty proficient in the basics…)
If you want to start working from home, think about what skills you have that you can do for other people long-distance. Are you really good at something computer-based (Web design, creating complicated Excel spreadsheets, selling things on Ebay)? Can you make something that people would buy? There’s all sorts of things you can do – you just have to find your niche and market yourself well.