Where online can I look up the standard deviation of a given stock or mutual fund?

Google Finance provides standard deviation and other statistical info for some mutual funds and this is very helpful, but it does not provide this information for all mutual funds or for any individual stocks I've checked. Is there a website that provides this for stocks and more mutual funds?

And/or where can I download the historical data to calculate it myself in Excel?

Calculate it yourself using excel.

A rule of thumb I follow, and I encourage others to do as well, is to do all of your own calculations. Beta, for instance, is NOT a fixed value...compute your own.

Not that I know of but you can calculate it fairly simply in Excel.

Standard deviation from what? The "ideal" stock would have an SD of 100% over any given time period because it would be rising constantly!

You might be thinking too hard. The day-to-day fluctuations in a stock's price have nothing much to do with anything. Even long-term trends are meaningless...(Enron trended smoothly & steadily upward until it went out of business!)

Use Excel or a calculator.

You can get historical price data from the market where the stock/fund is traded (nasdaq.com has good tools for this).

Checking out standard deviation could still be important to see how volatile the stock price is. However you should choose a relevant time period when you would measure it (one month, one year, etc.). It should be the same time period as the ones you got from Google Finance for it to be comparable.