Smashwords. They will get you into the Nook store, the iBooks store, as well as the other major ebook stores. They also sell ebooks, and you can create coupon codes for special offers. You can also download your ebook to send copies to reviewers. There's also DriveThruFiction, which sells PDFs. There are one or two others; I hope someone else will remember them better than I can at 7:30 in the AM.
Is it worth it? Yes! The more places people can find you, the better your chances are of selling more books. You also want to keep your options open. There were posts in the KDP Select thread about the risk of disappointing fans who don't own Kindles. Your sales on other platforms might not match Kindle sales, but you never know. Some books do better on one platform than the rest.
Make certain you read the Smashwords Style Guide. It will help you get your doc files ready for uploading. Have something that will let you look at mobi and epub files so your books will look right.
Good luck!