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http://www.reddit.com/r/sideproject

Note: I moderate r/SideProject. This subreddit has similar content to "Show HN," but each link spends more time on the front page. It is also a good place to test submission titles before posting to HN.

I've already posted my side project in another subreddit, is it bad form to post it in r/SideProject as well?

Submit the link to your project in r/SideProject. It is definitely not bad form, since it is a subreddit for sharing side projects.

Startupli.st, AngelList and MoMB (http://momb.socio-kybernetics.net/) come to mind.

Metafilter Projects can give you some good insight/test users, especially if your product isn't just techie-focused.