I'm using a Pi to run my home automation and I think it uses less power than the old system (Cytech Comfort). Running a whole Linux stack is overkill for a lot of application.
I may be setting up a solar system some time. Looking to build a garden shelter and solar could do the lights for that. I'll have to think about what other features I could incorporate.
For small applications, this may interest you. It depends on your power needs, of course, the chip mentioned outputs a max. 2A into the batteries, but the way it works is brilliant, a kind of poor-man's MPPT. Should be implementable using a microcontroller.