In Vim/Vi, how do you move the cursor to the end of the previous word?

Try ge:

ge                      Backward to the end of word [count] |inclusive|.

                                                        *gE*
gE                      Backward to the end of WORD [count] |inclusive|.

You may also want to do these to go to the space preceding a word (one character after ge: (both are only 2 keys)

1. FSpace will take you to the space after the previous word
2. Bh will also do that, in most cases.

The exception is when you are at the beginning of a line, in which case

3. k$ does something similar, in 3 keypresses

as seen on VIM manual (section 03.1), you can use ge to go to the end of previous word

Unfortunately it's not a single key... but ge is what you're looking for, I think.