He threw his plate onto the floor.
The band climbed onto the stage.
Nevertheless, some people still don’t accept it as part of standard British English (unlike into) and it’s best to use the two-word form in formal writing.
In US English, onto is more or less the standard form: it seems likely that this will eventually become the case in British English too. Remember, though, that you should never write on to as one word when it means ‘onwards and towards’. For example:
√ Let’s move on to the next point.
X Let’s move onto the next point.
√ Those who qualify can go on to university.
X Those who qualify can go onto university.