6v devices are fairly common; 12v may be more standard, but 6v may be all you need for a particular application - a radio, or bike lights. 6v sealed lead acid batteries are easily available, or five 1.2v rechargeable batteries in series can be used.
My bike lights are 6v; 6v lights designed to be permantly bolted on for dynamo used are often found second hand, and even new, and give a solid bright light, as essential for traffic as out in the countryside. In this case I use a 6v 4Ah battery charged om the mains, but this could instead be charged by other means if necessary, by a bike dymamo, even, if the charging voltage is limited to 7.2v.
The traditional 'bottle' style bike dynamo is designed to give approximately 6v AC at about half an amp; put this through a bridge rectifier, regulate where necessary and you have a mobile power source to charge batteries, phones etc.