Fringe scienceFringe science aspires to become mainstream. Most fringe science will never be legitimate, but some will. A good place to begin is Robert Park's Voodoo science which describes criteria for recognizing illegitimate science. We can also learn from formerly fringe sciences that have become legitimate.
Plate tectonics is a good example of the latter. Alfred Wegener (1912) suggested continental drift. The geological community noted the evidence, but rejected the model for lack of a mechanism. When sea floor spreading was discovered in the 1960s, plate tectonics became mainline geology and has proved intellectually productive.
Perhaps more interesting is high temperature superconductivity. The Bardeen-Cooper-Schrieffer Theory predicted that superconductivity was unlikely above about 20K. The accidental discovery of a new class of superconductors in 1986 dramatically changed the situation. Room temperature superconductivity would be valuable. Analogy to recently discovered superconductors has not lead to RT superconductivity, and seems unlikely to do so. RT superconductivity is not mainline science, but the situation could change.
Astrology is a fringe science that will never be legitimate. It violates Park's first criterion by appealing directly to the media, the 3rd by having effects at (below) the detection limit, the 4th by relying on anecdotal evidence, the 5th by claiming legitimacy through history and the 7th by violating known physical laws.
Cold fusion is less extreme. It violates Park's 3rd criterion by having small effects, the 4th because evidence is generally anecdotal and the 7th because calculations from known physical laws suggest it's impossible. Nonetheless it's an active research area with many published papers. If current physical laws don't fully explain fusion and if measurable and reproducible effects materialize, the situation could change. The economic potential of cold fusion encourages wishful thinking.
Intelligent Design is current fringe science. It violates Park's 1st criterion by making claims in the media (not in the scientific literature), the 2nd by claiming that the scientific establishment suppresses it, the 3rd by having having no detectable effect, and the 7th by violating known laws of nature. It also fails Popper's criteria for a good theory. To most scientists Intelligent Design has less appeal than cold fusion because it has no economic potential and because it's analytically sterile.