Generally, each community will regard one of its poskim as its Posek HaDor ("Posek of the present Generation").
Most rely on the rav in their community (in Hasidic communities, sometimes the rebbe) or the leading posek.
Poskim will generally not overrule a specific law unless based on an earlier authority: a posek will generally extend a law to new situations but will not change the Halakhah; see the article on Orthodox Judaism.
Conservative Judaism approaches the idea of posek, and Halakha in general, somewhat differently: poskim here apply a relatively lower weighting to precedent, and will thus frequently re-interpret (or even change) a previous ruling through a formal argument; see Conservative Halakha. Although there are some "poskim" in the Conservative movement - e.g. Rabbis Louis Ginzberg, David Golinkin, Joel Roth, and Elliot Dorff - the rulings of any one individual rabbi are considered less authoritative than a consensus ruling. Thus, the Conservative movement's Rabbinical Assembly maintains a Committee on Jewish Law and Standards, whose decisions are accepted as authoritative within the American Conservative movement. At the same time, every Conservative rabbi has the right as mara d'atra to interpret Jewish law for his own community, regardless of the responsa of the Law Committee.
Although Reform stresses the individual autonomy of its membership, it never completely abandoned the field of responsa literature, if only to counter its rivals' demands. Even Classical Reformers such as Rabbi David Einhorn composed some. Rabbi Solomon Freehof, and his successor Rabbi Walter Jacob, attempted to create a concept of "Progressive Halacha", authoring numerous responsa based on a methodology laying great emphasis on current sensibilities and ethical ideals. Full text collections of Reform responsa are available on the website of the Central Conference of American Rabbis.
The Reconstructionist position is that if Jews had formed cohesive communities again, their rulings would be binding, but presently Judaism is in a "post-Halakhic state". Therefore, their basic policy is to allow tradition "a vote, not a veto" in communal and personal affairs.
List of poskim and major works
In chronological order, by the year of birth, and if needed, secondarily, by year of death and surname.