Grammar

"Would have", not "would of"

Also applies to to "could have", "should have", etc.

"Than", not "then"

And vice-versa. A common misuse of either word.

"I saw", not "I seen"

"Seen" is the past participle of the verb "to see", and must be used in conjunction with another verb, for example, "I have seen".

"Nothing", not "notting"

This is a new one that's popping up about the place.

"less" vs "fewer"

fewer is used for countable objects, less is used for uncountable objects or quantity (50 euro or less - even though money is countable)