Most e-book sites excluding amazon have DRM-free books (see Fictionwise.com, ebooks.com, etc) but the presence of DRM is up to the publishers.
So use a publisher who doesn't have DRM or device lock-in, like Baen, Del Ray, Night Shade, SRM, Subterranean, and Tor (all at webscription.net). Their e-books are available in rtf, html, pub, etc. I'll point out that they also tend to discount vs. hardcopy as well as offering advanced reader (pre-release) copies for addicts.
Plus there are different kinds of DRM. I detest the Adobe and Mobireader approach that requires a remote server. External authentication or device-specific encryption == devil. You can't trust that you'll be able to get to your book when you want it.
The alternative is password protected encrypted files. The eReader format sets a password on your books based on your name and credit card number. As long as you have the info and the device supports the format you can read it on anything you want. The "safety factor" to the publisher is that if you post the book on a warez site they know who did it, since you also have to post the password (name+cc). Plus, you know, your credit information gets out on the net, which is almost punishment enough by itself.
If you're interested in an ebook reader check this wiki page on ebook formats that shows what hardware will read what format.
Based on that little tidbit, the Nook, and therefore Barnes & Nobles' ebook store, has the advantage that all their formats are either unencrypted or devoid of remote authentication. Maybe not ideal but at least you aren't trapped to a particular device or dependant on a remote server.