Westartup is an online community for entrepreneurs & investors. It allows you to create a page for your idea & gain feedback from seasoned entrepreneurs & investors.
Reddit - Find the right subreddits.
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.