If you want to self publish your ebook in as many electronic formats as possible, then Smashwords is probably the best bet.
In a nutshell they take your .doc document and turn it into up to seven formats for you. They call it the meatgrinder conversion process. And that's pretty much all they do. You can get an author's page and book pages, and you can embed your youtube book trailer, but when it comes to self publishing Smashwords is the least option rich offering.
Which is great if you already have your whole package together. If you don't need editing, a cover, book reviews, a physical book, etc... Smashwords is perfect. And since you can do Smashwords as well as other self publishers, there's no reason not to put your book on Smashwords.
So, how to get on Smashwords? First off start downloading. Get yourself a copy of Smashwords Style Guide. It's free and goes through every single step you need to do to get your book ready for Smashwords. Following this guide is important because Smashwords is going to take your document and make it readable by virtually everyone. So, if you want something that looks just as good on a computer screen, as a Kindle, as a Nook, as an iPod, you need to follow the guide.
I thought the Smashwords website was a tad clunky when it came to finding the information I was looking for. (Example: how do I get an ISBN? Hmmm... it's not in the publishing section. Typing ISBN in the search function brings up every book that has one, so that's not helpful. Finally I checked Dashboard, and it was there under ISBN manager.) But it is very easy for getting your book in online. Fill out the forms, download the document, download your cover, (You only need a front cover image on Smashwords because they don't sell physical books.) pick your category, write up your description and tags, and hit publish. Then wait while Smashwords does it's magic formatting, and eventually you've got an ebook.
Now to the part you really want to know about, royalties. Smashwords does a very easy royalty calculation, take the price of your book, subtract the paypal processing fee, multiply that number by .85, and you've got your royalty. Or, more plainly, your royalty is 85% of whatever is left after the processing fee has been removed. You can chose to be paid by Paypal or paper checks mailed to you. Payments are one month after any quarter where you've got more than $10.00 accrued, if you go with Paypal, or $25.00, if you go with paper checks. Quarters end in March, June, September, and December, so payments happen in April, July, October, and January.
Royalties get a bit more complicated if you go with the Smashword Premium Catalog. The Premium Catalog lists you with Barnes and Noble, Apple, Sony, and Kobo. Each of these retailers has their own royalty set up: for example, Apple does a 60% royalty. You control where your book sells, so you can opt in or out of any distributor you like. Unlike CreateSpace, you don't pay for the Premium Catalog for Smashwords, but you do have to have a properly formatted book with an ISBN, (A new ISBN, the one you used for your print book won't work, but they can hook you up with a new one.) a proper copyright statement, and a good cover image. But, I would hope if you are going to publish, you'd have all of those things already.
Unless you don't have a way to turn your manuscript into a .doc document, there's no reason to skip Smashwords. It's free. The royalty rates are the best I've run into yet. Formatting your book properly may take a while, but having it available for pretty much anyone, everywhere is worth your time.