By default, Haiku's option key, to invoke commands from menus for example, is not the usual CTRL key, but ALT instead. This has historical reasons, because the BeOS was inspired somewhat by MacOS. After you get used to it, it actually feels better as e.g. ALT+C and ALT+V is reached more conveniently on the keyboard and these commands seamlessly integrate into the bash shell of the Terminal, where CTRL+C quits the running process.

In any case, you can switch to the maybe more familiar CTRL key in the Menu preferences. The user guide will always describes the default configuration with option key being ALT.

General shortcuts

Here's a table of many of the most commonly used shortcuts that are always available, even if there isn't a corresponding menu:

ALT+Fx		Switches to Workspace X (Fx is the function key corresponding to that workspace).
CTRL+TAB		Hold down to bring up the Twitcher.
CTRL+ALT+DEL		Opens the Team Monitor.
ALT+ESC		Enters the menu bar (leave with ESC).
ALT+C		Copies selection to clipboard.
ALT+X		Cuts selection to clipboard.
ALT+V		Pastes the clipboard's contents.
ALT+W		Closes the current window.
ALT+Q		Closes the whole application.

Shortcuts for Tracker navigation

Additionally to the general shortcuts, here are more for navigating with Tracker:

ALT+CURSOR-UP		Opens the parent folder.
ALT+CURSOR-DOWN or RETURN		Opens the selected folder.
WIN-KEY		Holding it while opening a folder will automatically close the parent folder. This also works when navigating with the mouse.
right MENU-KEY		Opens the Deskbar menu (leave with ESC)
ALT+Z		Undo last action. The undo history is only limited by the available memory. Note, that this only works for actions on the file itself, changed attributes and permission settings can't be undone. Also, once a file is removed from Trash it's gone for good.
ALT+SHIFT+Z		Redo the action you just reverted with ALT+Z

Other key combinations

You can add or remove items to/from a selection by holding down a modifier key while clicking on a entry (or file in case of Tracker).

SHIFT		This will select everything between the first selected item and the one you click on.
ALT		Adds or removes the item you're clicking on from the selection.

In a Tracker window, if you just start typing, Tracker scrolls to and selects the file that best fits your incremental search. If there's no file starting with your typed letters, files that contain the search string anywhere in their name or other displayed attributes are selected. This search is not case-sensitive.
The letters you type appear at the bottom-left, where normally the number of items are listed. After a second it reverts back and you could start a new incremental search.