Jim send through BeThePro, a great example of a branded (or unbranded) community from Bosch.
A few things to note here:
1) It's unbranded. This means the community can attract both their customers and others. If you make the community about your brand, you immediately reduce the size of the possible audience.
2) It uses a simple platform. It's build on wordpress using BuddyPress for forums/groups. This allows for full customization of the platform.
3) Content from members. The majority of community content is provided by members. By giving members the ability to create content (even if it doesn't match the standards of the brand) they give high levels of ownership to the community.
4) Build a community around the blog. The community began as a blog and only once it has built up a significant following were community features added. By the time they added the forums in 2012 (a year after the site launched), there was a community of people ready to use it. This avoided the big launch syndrome.
5) Strong relationships with top professionals. To ensure a high quality of content, Bosch has invited many of the top professionals in the sector to contribute their advice. This provides a constant source of content and solidifies their relationships with those they want help from.
Building a community on behalf of an organisation is difficult. Most organisations take a top-down approach. They make the community about themselves. Buy an expensive platform. They drive huge amounts of traffic to the site. They hope that some of it will stick.
It's always better to make the community about the topic, use a simple platform, start small, and build relationships as you go.
As a bonus, you can see Vanessa's great list of examples here.