What are the best sited for work hour by hour like proofreading for example?
The high end ones, like Elance, are difficult for work hour by hour.
On odesk you can do that, but there are lots of fake accounts (not real persons) and its difficult to get applicants that have the required skills and addresses your needs.
Also it is little risk for them to take on a job they can't do. If they get a bad rating they can return the money and the rating is not visible.