I find myself wanting to develop certain projects, but most of the time I lack motivation because I develop by myself.
What I usually do is look for similar existing projects, and ask the developers if they like to collaborate, but it's rather hard.
Is there a good place (a website maybe) to find people that are interested in the same project as me, and therefore would like to collaborate?
You code by yourself?
Release the code on one of the open-source foundries. (code.google.com. sf.net. bitbucket.org , github.com ,etc...)
Pick an easy license (x11/MIT is good, GPL2/3/AGPL3 if you like, among others.)
Write simple instructions on how to deploy, run, with a one-page tutorial.
Have a website where you write about the stuff you build, and the stuff you'd like to build.
Find people who need some help and help them. Don't over-extend yourself.
It takes time to build trust. Trust takes time.
You wrote:
What I usually do is look for similar existing projects, and ask the developers if they like to collaborate, but it's rather hard.
If you see an open-source project out there, odds are the developers already like to collaborate. What they might not want to do is talk about grand schemes about how to turn the software into the next fifty billion-dollar behemoth. Generally, if you join the mailing list, introduce yourself ("Hi, I'm Joe, and I like to do X, and I like this software."), get and use the software, and provide feedback and constructive criticism, and demonstrate that you are following instructions or at least attempting to, and then, then, if you provide a patch (or a branch if github) it might be looked at and considered.
Do follow the project methodology. For example, if they use tests, submit tests with your patch, that sort of thing.