Great question, and I am sure the wisdom of Quora can point me to knew communities I don't know of yet.
However, let me share a few of communities I follow/contribute to.
Westartup - Westartup
Westartup is an online community for entrepreneurs & investors. It allows you to create a page for your idea & gain feedback from seasoned entrepreneurs & investors. The community seems strongest for early phase startups, but is working hard on developing the service further to ensure value for startups in later phases.
Reddit - Find the right subreddits
I only started experimenting with Reddit recently. However, I am really growing found of the many amazing communities formed around subreddits. For entrepreneurs I recommend you to participate in the following subreddits: /r/Entrepreneur, /r/Entrepreneur Ride Along, /r/Smallbusiness & /r/startups. Each of the subreddits are full of helpful & knowledge people.
The Quick MVP G+ Community - QuickMVP Community
The Javelin guys are doing something interesting with their QuickMVP product. Not only is it a very interesting approach to validating business ideas, they are also building a community of entrepreneurs around the service. Btw. you can find similar communities around many other services targeted entrepreneurs, and it is a good idea to join the communities around the services/products you use most (e.g. Wordpress, Shopify, Mailchimp...).
Facebook groups - Ask around to the next startup event & I am positive you will discover a "closed" Facebook community of entrepreneurs.
Hashtag communities - Around the world startup ecosystems are adopting their own hashtags to connect with each other on Twitter & other social media. (Together with Quora I'm building a list of those here: What are your favorite hashtag communities on Twitter?)
HackerNews-like communities: Whether it is Hacker News, GrowthHackers, Onstartups or Product Hunt, they all are very useful places for entrepreneurs.