I use a cordless mouse, and it's powered by a pair of alkaline LR6 batteries (the ubiquitous 1.5 V ones). It's been frustrating to have to replace them every few months once they drain, often in a middle of an activity (such as gaming online). Even though I try to recycle the used batteries, having to go out for fresh ones is an inconvenience.
So, instead of obtaining a stock of alkalines, I bought a few rechargable LiPo batteries in the 14500 form factor, and a lithium-polymer charger. The nice thing about this form-factor is that it matches AA: mechanically, you can insert 14500 in whatever AA holder your device has got. The bad thing is that the voltage is different. Single LiPo cell has 3.1–4.2 range (with 3.7 V nominal), while alkalines produce 1.0–1.5. A pair of alkalines, however, is usually required for the common 3.3 V digital logic; so, with trivial modifications to the holder, I was able to go like this:
