I want to have the ability to share the details of my holdings and have a website track the return over time while making this information 100% public to anyone who may be interested. What websites provide this capability?
This is going to be a bit of a shameless plug, but I've build a portfolio tracking website to track your portfolio and be able to share it (in read-only mode) as well.
It is at http://frano.carelessmusings.com and currently in beta.
Most portfolio trackers are behind a login wall and thus will lack the sharing function you are looking for. Examples of these are: Yahoo Finance, Google Finance, Reuters Portfolios, MorningStart Portfolios, and many others.
Another very quick and easy solution (if you are not trading too often) is a shared google docs spreadsheet. Gdocs has integration with google finance and can retrieve prices for stocks by symbol. A spreadsheet can contain the following:
Symbol, Quantity, Avg. Buy Price, Price, P/L, P/L% and so on.
The current price and P/L data can be functions that use the google finance API.
Hope this helps, and if you check out my site please let me know what you think and what I could change.