I only recently spotted this when using the on-screen keyboard on my Surface RT. The top line of keys (from q to p) have little numbers, from 1 to 9 and then 0, shown in the top-left corner of the corresponding letter key, but there’s no function or control key that appears to activate the numbers.
Instead, you just hold the letter key and after moment some other keys pop up. So holding Q will pop up 1, for example. Some other keys will pop up letter variants with accents. E for example will give you 3 and é and others. The punctuation keys also support this, so holding ? gives you access to #, !, etc. –The full stop, comma and quotes keys also do this.
Before this I was using the &123 key to switch into symbols mode to pick out punctuation, but this is much quicker and more intuitive.