Solar Panels and Battery Packs
Whilst the price of Solar energy isn't yet cheap enough to make it more attractive than pulling power from the grid, it's certainly now useful enough for situations where grid power isn't an option. My usage so far has been when camping and also when sailing - times when connecting to the grid isn't possible and the option for having your own power supply is attractive.
There's two main downsides to solar in my opinion; 1) You can only get it during the day. 2) The panels need to be carefully aimed at the sun to make the most of the power. 3) The current the panel produces isn't always enough to maintain the device in charge mode (especially true of iPhones/iPads etc)
Kind of obvious, but for my purposes this means that the panels must be used with a battery pack for maximum convenience.