opportunist

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

• noun One who takes advantage of any opportunity to achieve an end, often with no regard for principles or consequences.

from The Century Dictionary.

• noun [capitalized] In French politics, a member of that section of the Republican party which believes in regulating political action in accordance with circumstances, and not by dogmatic principles.
• noun In general, one who takes advantage of opportunities as they occur; one who waits for an opportune time before attempting to bring into practice or to urge upon others the principles or beliefs which he holds; one who makes the best of circumstances as they arise; hence, one who is without settled principles or consistent policy: opposed to extremist.
• Of, pertaining to, or characteristic of the political party known as the Opportunists; hence [lowercase], of or pertaining to opportunism, or the observance of a waiting policy; making the best of circumstances while waiting for a suitable time for the proper carrying out of one's views.

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

• noun Recent One who advocates or practices opportunism.

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

• noun Someone who takes advantage of any opportunity to advance his own situation, placing expediency above principle.

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

• adjective taking immediate advantage, often unethically, of any circumstance of possible benefit
• noun a person who places expediency above principle