Parasite, sycophant, favorite, toady, lickspittle, bootlicker, hanger-on, leech, sponge, sponger all signify a person who is supported or sustained or seeks support or sustenance, usually physical but sometimes social or intellectual, from another without right or justification.
Parasite applies primarily to a person who as a matter of policy is supported more or less by another and gives nothing in return, but it is often extended to anyone who clings to a person of wealth, power, or influence in order to derive personal advantage or who is useless and unnecessary to society.
Sycophant applies to one who clings to a person of wealth, power, or influence and wins or tries to win his favor by fawning, flattery, or adulation.
Favorite applies to a close associate or intimate of a king or noble who is unduly favored by him, especially with power; it may suggest parasitism or sycophancy on the part of the one favored and often connotes the exerting of undue or improper influence.
Toady, often interchangeable with sycophant, stresses more the servility and snobbery of the social climber.
Lickspittle and bootlicker are interchangeable in common speech with sycophant and toady, implying, however, even stronger contemptibleness.
Hanger-on applies to someone who is regarded, usually contemptuously, as adhering to or depending unduly on another especially for favors.
Leech stresses the persistence of clinging to or bleeding another for one's own advantage.
Sponge and sponger stress a parasitic laziness, dependence, and indifference to the discomforts caused and usually a certain pettiness and constant regard for opportunities to cadge.