I haven't searched the list exhaustively but I've already come across a few sites that make me feel foolish for presuming to assume that my 'great ideas' had not already been thought of and developed by others. For example:
Mobsaver has very similar functionality to the cellphone shopping aid that I envisioned in my Long Tail post last month. The use of SMS as the search string rather then a digital picture is perhaps a little more cumbersome (but also much easier to develop). Additionally the database of retailers it searches seems to be limited to 'just' amazon and ebay.
Carbonite is an online backup provider 'for the masses'. Fixed monthly price, unlimited storage spage. Doesn't have the same 'Lead User' draw that I proposed and isn't p2p, but it doesn't have to be in order to work.
Competitous isn't exactly a cross-medium trend watcher, but is a competitive intelligence manager that allows you to keep track of what your competitors are doing online. With some creative definitions of competitor, though, I could see how this tool could be used to track trends as they spread across entertainment mediums.
Finally in this post I learned that dogster (which I mentioned in passing recently) has raised one million in VC and claims one million a year in revenue. Hamster.com anyone? ;)