I have a lot of project ideas (like many of you) but I have some issues. There seems to be no easy way to find existing project online or to ask people what they think of an idea. I'm looking for a social project platform that allow people to search for projects, to contribute to them and to suggest new projects. It seems like SourceForge and FreshMeat are some of the best way to browse open source projects but the ability to bring new ideas and ask if there is a need for it seems limited since it was build to host open source projects, not help people start new ones and contribute to them.
Is there any kind of online platform that allow people to find, contribute and create projects online?
What your first commentair said. The thing is that "projects" are no common denominator for anyone. What binds people together is rather "skydiving" or "gardening". Projects just isn't an interest.
So, find what kind of area you want to get into, and see if you can find a forum or community for this online. Here you may find your innovators and early adopters, and from there you may be able to test your product.
For open source projects, I think sourceforge is leading the way.
On building your projects: Seeing as this is a website focused on startups, it's natural to assume you want to monetize some or all of your ideas - that doesn't mean their not community-driven or even open-source, but rather they have the funding to build a more sustainable project.
I think you'll find http://kickstarter.com to be a nice place to propose, measure interest and gather both interest/community and funding for projects.
On finding interesting projects: The question about finding projects is, as most people are saying, quite difficult. I suggest you take a look at popular freenode IRC network channels - for open-source projects - and start following some thematic feeds such as rubyinside 's for gathering news on specific areas of interest.
I find there are a lot of new projects, startups and ideas on Hacker News every week - that's worth following too.
Update : this seems like a website for exactly what you're searching: http://www.builditwith.me/