I have a financial spreadsheet that I use to track the distribution of my portfolio. I follow a system where I have a target allocation for each of a small number of assets, and then I buy or sell shares when any given asset gets beyond a certain tolerance of its target allocation.

Using Google Spreadsheets and the Google finance API available with Google Apps script, I was able to automate every part of this except syncing it with my account (since there's no API to access my financial records.) All I have to do is click a button, and the cells change color based on whether I should buy, sell or hold.