Nook for PC - http://www.barnesandnoble.com/u/nook-for-pc/379003591/
Nook for Mac - http://www.barnesandnoble.com/u/nook-for-mac/379003592/
Kindle for PC - http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000426311
Kindle for Mac -http://www.amazon.com/gp/feature.html?ie=UTF8&docId=1000464931
You can obtain e-books for free in one of four ways:
(1) Check out e-books from a library. Even if your local library doesn't carry e-books, you may have access to a state-wide e-book library.
(2) "Purchase" books for free from Amazon or Barnes and Noble. On Amazon, you can look up the top 100 free Kindle books and then sort by genre. On Barnes and Noble, you can perform a keyword search for a genre and then sort by price "low to high" to find free books. Barnes and Noble also offers a page that shows free books by genre, but the page doesn't list all genres. Sometimes books are only offered for free for a limited time period, so check back often. Also, always check the price before downloading/purchasing to make sure the book is still free. In most cases, downloading should be free, but be sure to check on possible fees when downloading outside of the United States, or similar circumstances.
Before I purchased a Kindle, I tried out the Nook and Kindle PC programs on my computer. Please note that you usually have to keep a credit card on file with Amazon or Barnes and Noble if you choose to download from them. That way, if you decide to buy a book from them, you can do so easily. If you feel uncomfortable using your credit card on the Internet, please consider checking out e-books from your local library. However, I would encourage you to check out your credit card's policy for online purchases. In some cases, card members are not held liable for unauthorized purchases if reported to the credit card company (per cardholder policy--be sure to read the policy for your card or speak to a customer service representative). Also, be aware that shopping offline doesn't protect you from identify theft. My state lost a hard drive with taxpayers' social security information on it. Also, before online transactions, identify thefts stole carbon copies of credit card receipts. So you don't have to go online to have your identity stolen. Don't be afraid to go online. Just be cautious and use good judgment, just as you would when you make purchases offline.