Binary is easy to count, but a pain to decipher unless you know binary very well.
Each finger and thumb is a single digit (literally!) and can either be a 0 or a 1. I start with the thumb on my right hand, but you can start however you’d like.
Thumb up for 1
Thumb down and index finger up for 10, which is 2 in base 10.
Index and thumb up for 11 (3)
Middle finger up, index and thumb down for 100 (4)
Middle and thumb up, index down for 101 (5)
Middle and index up, thumb down, for 110 (6)
All three up for 111 (7)
Ring finger up and all others down for 1000 (8)
Etc.
All ten up, 1111111111, is 1023.
If you need to count a large quantity but don’t need to be able to quickly decipher it, this is useful! But I almost never use it in favor of the easily-understood 0-99 counting.