I second the comments above. In my experience, Unity 3D is primarily used by enterprises for promotional materials -- they build an experience, and then they publish it on the web. This could be marketing games, campus tours, product simulations, etc...
Unity 3D is a development platform. You need developers to create anything in it.
OpenSim is a good fit for someone who wants a Second Life-style virtual environment, with in-world building tools, avatars, inventories, etc... but with better controls, backups, and lower-cost land. Anyone can com e in and build, and there's a wealth of content available -- free OARs, Linda Kellie freebies, Kitely Market, hypergrid stores on many grids, etc... Developers aren't needed to create an environment.
There are some overlaps, though. Jibe and SecondPlaces are two products that try to build a virtual world-like system on top of Unity 3D.
Another option,if you're a developer, is WebGL and HTML5. You need a modern browser - Chrome or Firefox -- to visit these worlds but, on the plus side, you don't need to download any software or install any plugins. It just works.
And I just got off the phone with Douglas Maxwell, and the U.S. Department of Defense has built an entire free, open source virtual environment framework on top of WebGL -- https://virtual.wf
So if you're leaning towards Unity 3D, take a look at that, as well.
____________________________________________________ Maria Korolov • <a href="tel:508-443-1130" value="+15084431130" target="_blank">508-443-1130 • [hidden email] Editor & Publisher, Hypergrid Business
The magazine for enterprise users of virtual worlds.