Proofreading
Because we're all only human and no matter how many times you read your own work, there's bound to be a pesky typo that gets through.
Sometimes it helps to have a second pair of eyes on what you have written, especially if it is going to be published.
I have been involved in proofreading and editing for many clients, including the Institute of Physics and the United Nations.
I also work with corporates in other countries, when they need to make sure that their English language publications sound as though they were authored by a native English speaker.
The degree of editing can vary from a simple grammar and syntax check to a complete overhaul of tone of voice. It can be hugely satisfying taking a 'bank-manager' tone and giving it a warm, friendly feel.