If you're offering multiple formats, the easiest solution is to offer your ebook as a PDF through your website. Offer other formats elsewhere with a convenient link so people know where to go if they prefer something other than PDF. Most e-reading devices will accept PDF files.
In order to sell your book, you'll need a shopping cart. E-junkie.com is a user-friendly interface that can be used in conjunction with your site. Simply sign up, upload your files, and set your price and autoresponders (automatic emails that let customers know their transaction has processed properly and how to download their purchase).
For formats other than PDF, try smashwords.com. Smashwords allows you to distribute your ebooks for any price you choose through the Smashwords site. They have a file converter that takes one source file and converts it to multiple formats. This is accomplished through their Smashwords Style Guide, which gives you tips for simple formatting that will cross over several platforms without any of them looking too unappealing in the final analysis. Smashwords also offers premium distribution channels for books that meet their premium formatting and cover guidelines. Premium distribution will get your ebook into Barnes and Noble, Sony, Amazon and Kobo Books (formerly Shortcovers).
If your ebook is free, you have even more options for distributing your ebook. Free-ebooks.net allows you to post your book for a nominal fee and has a huge audience. Other sites include Scribd.com, Obooko.com and manybooks.net, which allow you to post your book for free.
Each of these sites has its own set of rules for upload but generally it involves filling out a form and uploading your file, or else sending the file to a site administrator for addition into the catalog.
Don't forget to put it on your own site. Without an interface like e-junkie.com, you'll have to put the file directly on your site for download, or else you can simply direct people through your site to your preferred download location. The simplest way to put a PDF download directly on your site is to use wordpress.com (They will host your domain name). Wordpress.com is a free blogging tool but many writers use it for their author website since many of the templates allow multiple pages in a menu bar across the top. When making a post you can upload media including PDF files, creating a very simple way to offer your ebook for free download from your site. All the reader has to do is click on the link created by wordpress.