Apparently Apple has more cash on its books than combined market values of Hewlett-Packard, Dell, Nokia, Yahoo, Sprint Nextel and Research In Motion.
Where do companies like Google and Apple keep their cash? Large banks?
You can find out the general types of investments by reading the public corporation 10-Q report that is filed with the SEC it can be accessed via the EDGAR system.
It will not tell you what securities they have, but it does identify the short term and long term investments categories and their value.
They are using several banks, hedge funds or other financial institutions, in order to diversify the risk inherent to the fact that the firm holding (a fraction of) their cash, can be insolvent which would makes them incur a really big loss.
Also, the most available form of cash is very often reinvested everyday in overnight*products and any other highly liquid products, so that it can be available quickly if needed. Since they are aware that they are not likely to need all of their cash in one day, they also use longer terms or less liquid investments (bonds, stocks, etc..).
For tax optimization, cash is stored mostly overseas, according to the New York Times.
For Apple, everytime a song or an app is bought in Europe, Africa or Middle East, money flows to iTunes Sàrl, in Luxemburg. Royalties on patents flow internally from Apple in California to Apple in Ireland. Then profits flow to the Carribean.
The problem is that cash cannot be brought back to the USA without huge taxes.