Most readers will be aware of sites like Squidoo. Their characteristics vary, but they are usually like a combination of blogging platform and article directory, and they allow users to make some money through various revenue sharing arrangements. Because they have so much content up there and are updated so often they rank highly in the search engines and are great places to build backlinks to other sites.
With these factors in mind I recently submitted a couple of lenses to Squidoo to get a bit of a feel for it. While they haven't been up there long enough to discern any noticeable SEO benefit to the sites I linked to (with the big arrow!), I do know that both have been indexed by Google.
Surprisingly, it does take a little while for the indexation to occur, even if you bookmark your lens here and there. And the process often seems to be twofold. Firstly, you have to wait a few days before you see the lens listed. When you do see it there, it often appears quite high in the results. Then it disappears for a few days more while Google does a bit more "thinking" about how it should rank. You can find it again at this stage, but it's often much further down the list of results. (This exact process occurred with one of my lenses, and I've read it described repeatedly on blogs.)
The reason I mention this is because many people submit lenses and freak out when they can't find them by Googling, even after a week or more. So, don't worry. It will happen.
That idiosyncracy aside, there's something very appealing about the whole lens building process. It's quite different to submitting articles to directories. With these, you just put your article into the form provided. But Squidoo uses a module based structure, and you can add text or link lists or other things. Just the fact that these options are available is conducive to creativity. While you're playing around with them, new ideas seem to suggest themselves.
The modules also make you think carefully about what you'll include, and pare the content down to the bare essentials. This means that lenses tend to be more jam packed with information, giving more value to the reader.
Another site that is a lot like Squidoo is Wikinut. You can make some money from it. However the general perception from people using it for this purpose seems to be that the financial fruits are very small indeed. Still, it has a nice design, a PR of 3 and it allows you to put dofollow backlinks in your articles. So it's worth contributing to for the SEO benefits alone.
Like Squidoo, Wikinut has a module based structure (although with not as many different options). So, that's another positive aspect. Also, it gets indexed very quickly. I've submitted two pages there so far and they both appeared within hours of being published, maybe sooner. So, it actually beats Squidoo in this regard.