Taoism and folk religion have thousands of gods; the following are the most prominent.
Jade Emperor 玉帝
The Jade Emperor (Yu Di or Yu Huang Shangdi 玉皇上帝 in Chinese) is believed to have created the universe, fashioning the first humans from from clay. He now rules over mankind, heaven and hell, and he's a good example of an interesting phenomenon in folk religion: The higher a deity's position in Taoism, the less often he seems to appear in temples. Despite having been a god for all eternity, the Jade Emperor has a birthday, the ninth day of the first lunar month.
Yellow Emperor 黄帝
A legendary ruler of the Chinese people and supposed ancestor of the Han race, the Yellow Emperor (Huang Di in Chinese) is believed to have reigned around 4,700 years ago after unifying tribes living close to the Yellow River. When he was well over 100 years old, he rose to heaven; ever since he has been revered as a major deity and the putative inventor of the wheel, medicine, armor, boats, the compass, the calendar and coins. His empress is said to have invented sericulture.
Guan Gong 關公
One of the most widely-worshiped and highest-ranking deities in the Chinese and Taiwanese folk pantheons, Guan Gong is easy to recognize thanks to his red face and the halberd-like weapon (known in Chinese as a guandao, 關刀) he invariably grips. Also known as Guan Di (關帝) and Guan Yu (關羽), he was a general who died in 219AD. Little is known for sure about his life, but it's said he was executed along with his adoptive son after being captured in the wake of a battlefield defeat. In houses of worship - including Tainan's Sacrificial Rites Martial Temple - Guan Gong is often flanked by statues of his son, Guan Ping (關平), and Zhou Cang (周倉); the latter committed suicide on hearing of Guan Gong's death. Businessmen, police and gangsters are among Guan Gong's keenest devotees.
Baosheng Dadi 保生大帝
Several centuries ago, China's south and southeast were pestilential. Medicine deities and plague gods were thus of great importance, and very few became more prominent than Baosheng Dadi, the cult of whom was brought to Taiwan by early Fujianese settlers. Known as Wu Tao (吳本) during his lifetime, he worked as a physician and is credited with once bringing a dusty skeleton back to life. After his death he gained a reputation for interceding in what seemed to be hopeless cases, and attracted a devoted following that's evident today in shrines like Dalongdong Baoan Temple.
Lord of the North Pole 北極大帝
Alternatively known as the Emperor of the Dark Heavens (玄天上帝) or Xuan Wu (玄武), this martial god (Beiji Dadi in Chinese) has a sizable following in Taiwan with at least 250 temples to his name, including Tainan's Temple Of The Lord Of The North Pole. Depictions of this god almost always include a snake and a turtle. Some say they were demons he subdued in battle. Another explanation is thoroughly gruesome: Many followers believe that, when confronted by Guanyin about his violent past, he showed contrition by cutting out his intestines and stomach. The former became a snake, the latter a turtle. Since then, the two creatures have been his faithful servants.
Also know by his Sanskrit name Kshitigharba, Dizangwang (地藏王) is a Buddhist bodhisattva usually worshiped by relatives of the recently deceased. Often described as the King of Hell, he doesn't delight in condemning souls to eternal torment, but rather intercedes to lessen the time the newly dead must spend in purgatory. He appears in many shrines, notably Dongyue Hall in Tainan.
Land Gods 土地公
In terms of visibility and sheer numbers, land gods dominate the folk pantheon. Each neighborhood, be it rural or urban, has one, and he's believed to use his powers to protect the district and its inhabitants from harm. Some land gods are worshiped as subsidiary deities in large temples - usually under the name Fude Zhengshen (福德正神) - but many have their own shrines. These are typically hut-sized, but may be as a small as a dog kennel. Land-god icons usually depict him as an elderly man in the traditional attire of a mandarin; oftentimes he carries a staff (a symbol of office) or a gold ingot (because he doubles as a god of wealth). In Hakka areas, land gods are known as Bogong (伯公). The birthday of a great many land gods is celebrated on the second day of the second lunar month.
City Gods 城隍爺
Called Chenghuangye ("Lord of Wall and Moat") in Chinese, town and city gods oversee distinct urban districts, but these seldom match current local government boundaries. In central Tainan, for instance, there are three city-god temples within 2km. Like some land gods, a number of city or town gods are believed to have been local notables who were posthumously deified. Especially lively city-god shrines can be found in Taipei, Hsinchu, Lugang, Chiayi and Tainan. Wherever a city god is worshiped, you'll also find an altar devoted to his wife.