There's also a free one from Network Empire, called Simple Silo. Themewise there's Ultimate Silo Studio. I've heard a few other themes mention it, but that's the only one dedicated to it. As for which plugin is better? Depends on what you're trying to do with it and how deep of a silo you want. Simple Silo makes it easy to do Page-->Post siloing. It'll create a menu widget that autolinks within the silo and gives you a few shortcodes you can manually place if you want. WPSilo2 does Category->Page->Post siloing with Simple 301 redirect added in so that the category pages silos redirect to the Page level silos. It's also nice because it gives you a little bit more control over the styling of the pages it creates.
I think I'm just going to manually do my site with a redirect for the Category to Page connection and then using custom menu widgets in the sidebar with Jetpack's widget visibility feature. This way I can have a little more control over how links are placed.