I prefer MOBI Pocket Reader. Its free. It allows you to change the text sizes and it remembers where you last left off in the book.
Get it here :
http://www.mobipocket.com/en/Downloa...ailsReader.asp
i also use Calibre to convert to mobi format. Its also free and it can convert most files to whatever reader format you want.
Get it Here:
http://calibre.kovidgoyal.net/download