(Click the image above for a closer look.)
Just launch the application, create a new portfolio, and start adding the stocks you're invested in along with any transactions. Portfolio Viewer autocompletes your stock name and ticker symbol when you're adding an investment, then automatically imports stock values over time, calculates your annual rate of return from your portfolio, determines your investment allocation, and highlights the diversity of your investments in the allocation grid.
I wouldn't count on Portfolio Viewer to help determine all my investment moves, but it's a great little tool to keep an eye on the performance of your portfolio.
Portfolio Viewer is freeware, requires Adobe AIR, and works on Windows, Mac OS X, and Linux.