I have an idea for a new plugin.
Where can I propose the idea for the plugin so that someone might pick it up?
p.s: I won’t be willing to pay for it (it is not that important to me), but I still think it is a cool idea. I am also not going to try to make it myself, since my PHP skills are not good enough.
I’d suggest jobs.wordpress.net. It’s a job listing site for freelance WordPress development. Just be very clear that you aren’t offering any pay for the project.
Alternatively, you can also suggest things in the official WordPress forums: Requests and Feedback.
One third option is to mention it to someone on the WP-Hackers mailing list.
In addition to what @EAMann said consider adding the request here on StackExchange’s WordPress Answers in the form of a question. Describe the plugin and ask if anything similar exists.
The other place to suggest things is the WordPress Ideas site. Really it is a place to request core functionality, but if you posted an idea there it could accumulate stars which might convince a plugin dev to take it on.