I use Google Finance to track stock/fund portfolios. Many of these portfolios track some investment policies and I enter the actual trades in there too. But the portfolios keep showing the old (i.e. closed) positions in the table making it look cluttered. I checked on their product blog/forum and this seems to be requested by some people (and ignored). Here is a small bookmarklet which will do this. You will need to drag the link to your bookmarks bar (Tested only with google chrome). Next, when you are on the google portfolio page, just click on the bookmark and it will hide all the closed positions! Woohoo!
Drag this link to Bookmarks bar: GFinance: Hide Closed Positions
P.S. If there is interest, I can turn this into a google chrome extension (greasemonkey) so this gets done automatically when you go to google portfolios page.