I've just come across a new OS for the Kindle, and I wanted to pass along a warning. Save your money.
KindleOS X (4.1) is simply Duokan, the open source Kindle firmware under a different label. The KindleOS people didn't even recompile from source or change any file names. It's a straightforward copy of Duokan.
FYI: Duokan is a replacement firmware for the Kindle that lets you read Epub/Mobi/AZW, download RSS feeds, and do a bunch of other things. It's been getting incrementally better since I first came across it back in November, and I'm pretty sure it will one day pass the Kindle in terms of features. (You can get an English language version here.)
While I don't think that the KindleOS is worth the money, I will say that the KindleOS people have actually contributed something. Their version is a lot easier to install. They've reorganized the files and provided clear instructions. Their version might actually be worth the $5 for a beginner. You can find it here.
On the other hand, they are a bunch of liars. Here's a few of their claims:
What if we were to tell you that we have designed and released an amazing, completely new operating system that works on your kindle without changing or deleting your current Amazon Kindle OS?
We request a small donation of $4.99 (£2.99) for the KindleOS. This will enable us to work on the future versions of the Operating System and bring you the very best experience using your Kindle.
Folks, they can't recompile Duokan because (so far as I can tell) the source code has not been released.
In fact, I'm pretty sure that Duokan has not been released under any type of CC or GPL license (I can't find a mention). I'm pretty sure that KindleOSX is not just a copy but a pirated copy.
If you want to try Duokan, the link I provided for the free copy of Duokan has pretty good instructions. They're rather complicated, though.
And as always, this might damage your Kindle so use at your own risk.