Since searching the forums for a quick and easy answer is often not (quick or easy)......
I second Aldiko. It will read most of the popular formats.
Some good sites for free ebooks.....
Baen Free Library
(mainly sci fi and fantasy)
BaenCD at the Fifth Imperium
(over 100 free baen published books by their top authors. If you have ever ventured down the sci-fi row at B&N you will recognize these books!!)
Also, you can use the kindle app and purchase books directly from Amazon. I buy from Amazon, but since I prefer Aldiko for reading, I strip the DRM from the kindle books on the PC and port them over to Aldiko.
There are lots of sites and discussions about ebooks. If you need more info, feel free to PM or email me directly. Some of the stuff skirts the "strictly legal" line (some argue my removal of DRM is wrong...its NOT! I bought the book)
so its easier to do via PM or email.
good luck and enjoy your reading.