Just follow the links and you will find a wide selection of free downloads that can be used to read ebooks on a device you already own.
Adobe Digital Editions
This is a special e-reader program that can be downloaded for Macs or Windows.
Barnes & Noble
They have free apps for smartphones, tablets, computers, and the web.
They have free apps for smartphones, tablets, and computers
They have free apps for smartphones, some tablets, and computers
They have free apps for smartphones, tablets, and computers.
Each company has several devices so you should shop around to decide which will work best for you. I know a few people who find it difficult to read off a computer for any length of time. If that is true for you then I would suggest you try the ereaders that are front illuminated because they are easier on the eyes. Some devices work more like a tablet and allow you to surf the net while others only allow you to download books from their website.