eeeeeh strange question actually. If you want to submit change for the linux kernel, you have to subscribe to the mailing list (detail here : http://www.kernel.org/pub/linux/docs/lkml/
If you want start a new open source project then it's up to you, you might want to create an account on sourceforge.net. If you want to propose new features to a certain program, you should probably look at the program homepage or to mail the author directly.
What exactly are you searching for?