Any random laptop can probably be used for a decent amount of time, as long as the battery is already charged in advance, but the devices I usually use are:
* OpenPandora: a gaming handheld with a decent keyboard, GNU/Linux OS and a great community. It is small, but surprisingly efficient. I don't think they sell it anymore, but they are working on a successor and it's still supported by the producer. It has a *huge* battery for its size and only needs recharging from the solar panel when using wifi to get on the internet.
* Efika MX Smartbook: an ARM based netbook which came with Ubuntu from the producer: extremely light and with long battery times, Debian worked just fine on it. No longer produced, no longer supported and no longer upgradeable since there is no support in mainline kernel. Still works, sort-of, but you can't really trust it on the internet. It also was never really able to play videos under Debian because of the lack of proprietary codecs.
Nowadays I'm looking at the Arm64-based laptop that Olimex is working on: right now there is no mainline support, but AFAIK the sunxi community may be working on it. They are an established company, so they won't need crowdfunding, but there are still no infos on pricing etc.
laurelrusswurm, Claes Wallin (韋嘉誠) likes this.