Idea 1: Lost
A collection of lost items. Visitors would submit an image, drawing or written description of an item they've lost. Do they wish for it to be found? Do you recall the moment you lost it? Content would be sent by e-mail via a form on the site. Ideally, a collection of ~50 - ∞ items would be submitted.
Idea 2: Ingredients
A site where visitors would list an ingredient to cook with. It can be anything: meat, vegetale, a herb, a spice, a brand-name sauce. The items would be listed on the site, so no ingredient is submitted twice. Once the "gathering" has occured, users would be invited back to the site to create receipes using the ingredients that were submitted by the community. PHP would be used to gather submissions, so that they are instantly live on the site. Again, a collection of ~50 - ∞ would be great.
Idea 3: Carl Jung's Adjectives
1. Colour: three adjectives that describe it
2. Animal: three adjectives that describe it
3. Room: three adjectives that describe it