When I first got on the Internet, all this stuff was free. I hate searching for lessons online now because the top search engine results inevitably are these for-pay lesson plan sites. I abhor such commercialization.
I believe education, and indeed humanity, would be better served by open content wherein educators share their lessons for free using Creative Commons licensing. Karen Fasimpaur has written much on this topic. I've also blogged on the subject.
Right now, I am creating on-line test prep materials for fifth grade students in NY State on a wiki with Creative Commons Attribution Non-commercial Share alike licensing. I'm doing it for my own students--why not share it with everyone?
Think how the landscape would change if we all did this?
I agree with Steve about the commercialization. Did you find a pricing structure for this site? The practice of having browsers sign up for a free trial and then rolling over to a credit-card subscription seems sketchy to me.
If you want to find lessons, then such collections might be useful. If you want to post lessons for a community to use and refine, I agree that a wiki might be a good choice. A template can designed in wikispaces so that the all lessons can share basic structure.
I have started a wikispace for Rhode Island science teachers to be able to clarify the meaning of our state standards. Eventually this process might be a place to collect lessons that address these standards.
As Steve mentioned, if you're willing to share your lessons with others, a site that includes a Creative Commons license is a great way to go. One of the best is Curriki, a site for sharing lesson plans under an open license.
Another option is WIkispaces, a wiki site that includes a CC license on all content. (I use this site a lot, but one downside for lesson plans is that it's not really a "destination" site for folks looking for lesson plans.)
Finally, regardless of where you post (or even if you just distribute in print), you can always add a CC license that tells others they can use your resources. The simplest way to do this is to just write "licensed under Creative Commons Attribution" on the site. Or if you want to get fancier, use the CC license wizard. This tool will give you a CC piece of artwork and also some code that will get your lessons into CC search engines.