hanger-on

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who spends time in the company of another person or of a group out of admiration or for personal gain.

from The Century Dictionary.

• noun One who hangs upon a person, company, etc.; one who clings to the society of others longer than he is wanted; a dependent; a parasite.
• noun In coal-mining, the man who runs the cars or trams on to the cages and gives the signal to hoist.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who hangs on, or sticks to, a person, place, or service; a dependent; one who adheres to others' society longer than he is wanted.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who hangs on, or sticks to, a person, place, or service; a dependent; one who adheres to others’ society longer than he is wanted. - Oliver Goldsmith

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who persistently (and annoyingly) follows along