accessibility - Patterns for keyboard accessible drop down lists

Are there any standard keyboard shortcuts for making drop down lists appear? (in any kind of software, not just web based.)

I'm going to write some JavaScript to make the drop down lists (select and option tags) in a web app accessible via the keyboard.

I'm aware that Up Arrow and Down Arrow can change the selection, and pressing a letter key will cycle through the items starting with that letter (or typing a pattern will jump to the item starting with that sequence). But as selecting options in the list may change the form in this app I'd like to have a way to open the drop down so you can see the entire list and pick one of the options whilst viewing the entire list (or as much as will fit on screen).

Update:

AHA! Found something that works for some browsers.

When pressing Up Arrow or Down Arrow drop downs stay closed in web pages on Chrome, Firefox and IE9, and in those browsers settings dialogs.

However, in Firefox and IE pressing Alt+Up Arrow or Alt+Down Arrow opens the drop down for web pages and settings dialogs. It doesn't work in Chrome but there is a workaround. There is also a bug for chrome: Not able to open view/open drop-down list using keyboard

Also, the ribbon UI and settings dialogs in Excel 2010 do open the drop down when pressing just the up/down arrow keys.

Are the Alt+Up Arrow / Alt+Down Arrow shortcuts widely know/used? I'm debating whether to just add a polyfill for Chrome or write something to mimic the Office Ribbon behaviour in all browsers.