If you like Science Fiction/Fantasy, try this:
Baen Free LibraryAlso, if you subscribe to the Tor newsletter, they include one or two short stories every week. You can add "send to Kindle" to Chrome browser, then open the link to the story and send it to your Kindle. :C
The "send to Kindle" is handy, I've used it for trip notes, receipts, recipes...
Right now, my Amazon library supports 11 devices, down from a high of 13. ;) Hubby and I each have an e-ink K3, a Fire HD, a first gen Fire and a kindle app on our smartphones. I have a Kindle app on my Windows 8 computer. My mother and my bro in law each have an e-ink Kindle I maintain. I don't believe there is a limit to Kindles on your account, but some books are limited to being on no more than 6 devices at a time.