I use MobiPocket Reader to read most ebooks on my laptop and I have come to a firm conclusion that it’s the best ebook reader that I have ever used. A great feature of MobiReader is that Mobipocket ebooks can easily be synchronized between the computer and reading device.
There are some sites where you can download MobiPocket ebooks free of charge:
www.ManyBooks.net - It is a great resource where you can find public domain books. The site supports the download of these ebooks in several formats. Mobipocket ebooks is one of the formats supported in this great website.
www.Free-Ebooks.net - This site has a huge database of ebooks on various subjects. This site support downloads in MobiPocket format.
www.Diesel-Ebooks.com - Diesel Ebooks is well known for it's extensive collection of premium ebooks on their online store. Howvever, it also maintains a collection of over 400 free ebooks in Mobipocket format.I highly recommend you check out their website.
I will post more sites as soon as I lay my hands on them.