From A Wiki of Ice and Fire
A Maegi is a woman of Essos who practices blood magic. Blood magic is very powerful, but enjoys an awful and possibly well-deserved reputation. It is hated, mistrusted and forbidden by the Dothraki, who claim that the Maegi lay with demons. They may sometimes go so far as to kill anyone even assumed to be Maegi, as to prevent said Maegi from 'cursing' them with evil magic. Maegi often show a remarkable capability of telling one's future, usually from a drop of blood. According to Mirri Maz Duur, the word means "wise".
Some people reputed to be Maegi in A Song of Ice and Fire are: