Ilmare
The maid of Varda

 Eonwe
The herald of Manwe

 Osse
A Maia in the service of Ulmo, master of storms and sea gales. Once he was taken in by Melkor’s promises, and in the old days his temper was unruly, his sudden fits of rage threatening to sink mariners’ ships. It was mainly due to the soothing influence of Uinen his spouse, that Osse saw the wrong of his ways and returned to his old allegiance to Ulmo.

Uinen
Uinen is perhaps one of the best known Maiar. Being the spouse of Osse, she was the only one who could keep his fiery temper at bay. When Osse would raise waves that threatened drowning to mariners’ ships, she would alleviate his anger, and was therefore loved and respected by the Elves, seafarers in particular.

 Sauron
Sauron (also known as Gorthaur, Annatar, Artano, Aulendil, the Necromancer) was a Maia and he was Morgoth´s foremost servant. When Morgoth was imprisoned by the other Valar Sauron took control of Angband and Morgoth´s armies. After the War of Wrath, he took the name Annatar and became friends with the elves. Together they made the rings of power.  Sauron betrayed the elves and made the one ring to rule over the others, "One ring to rule them all, One ring to find them, One ring to bring them all and in the darkness bind them". By nine of these rings, he ensnared nine Edain kings of renown, and made them into his most feared servants - the Nazgul. He was the one who corrupted most of the Numenorean population into worshipping Morgoth and tricked Ar-Pharason into waging a senseless war against the Valar. His body was destroyed when the Valar destroyed Numenor and his dark spirit returned to Mordor. There he gathered his armies of all the dark creatures that had escaped doom in earlier days. Mordor then allied itself with the Easterlings, the Variags and the Haradrim. Since that time, he has been referred to as the Second Dark Lord. The Last Alliance of Men and Elves waged war against Sauron and his allies in the end of the Second Age. The Siege of Barad-Dur, Sauron's fortress, lasted for seven torturous years. Aragorn's ancestor Isildur took the ring from Sauron but lost it in the river of Anduin. Sauron rose to power once again in the Second Age and started the War of the Ring. When the ring was destroyed in Orodruin Sauron was deprived forever of his body and his power, becoming a mere spirit doomed to gnaw itself in the dark because of his inability to reincarnate.

 The Balrogs
Balrogs (Valaraukar) are rare creatures, enormous in size and extremely dangerous in combat. The most famous Balrog is the one who faught against Gandalf in Moria (LotR). They are maiar who followed Morgoth instead of the light. Balrogs use magical flaming whips and often combine this deadly weapon with a sabre. Durin´s Bane was the name given to the Balrog of Moria by the dwarves, called the Flame of Udûn by Gandalf. Gothmog, the son of Morgoth and the troll Fluithuin (Ulbandi); slayer of Fëanor, Fingon and Echtelion; lord of the Balrogs and high-captain of Angband was a mighty balrog who was slain in the Beleriand wars.

 Melian
Melian the Maia, spouse of Thingol and mother of Luthien Tinuviel, was more closely attached to the lands beyond Valinor than any other Maiar. Together with her husband she dwelt for many years in Doriath, which she surrounded by the Girdle of Melian, a protective wall of magic that was meant to turn out any intruder intending to enter the secluded realm.

Melian was known for her wisdom and her beauty, and she was skilled in herb-lore and various kinds of spells. Her heart was one with nature, an, being Yavanna’s disciple, she took delight in it, and cherished every living creature of her woods. Her companions were always the nightingales, who she is said to have taught to sing.

When Thingol was slain, Melian was allowed to return to Valinor, where she can be one day cured of her grief, and after her departure the Girdle of Melian failed, and the sheltered realm of Doriath was no more.

Olorin
Mentioned in the chronicles as the wisest among the Maiar. It is likely that Olorin is one of the aliases of Gandalf the Grey. 

The Istari
The Istari were an order of wizards created among the Maiar of Valinor who were capable and willing to come to the aid of the Free Peoples of Middle-earth in the Third Age. Originally the order included seven wizards: Saruman, Gandalf (Olorin), Radagast, Alatar, Pallando and two other mages whose names have not been recorded.

All the Istari were to a certain extent concerned about the doings of Men and Elves and attempted to resist the evil of Sauron, Gandalf more than any other, although some of them ended up pursuing their own ends or even betraying their cause. Saruman, who once was head of the White Council and the most skilled of the Istari where magic was concerned, succumbed to evil and fell under the dominion of Sauron. The two wizards whose names have not been preserved are said to have perished in the depths of Rhun or Khand. 

...The Others
Owing to the fact that the definition of ‘Maiar’ is so broad, it comprises a great many spirits with varying powers and abilities. Some of them were quite distinguished, while others hardly left a trace in the history of Arda.

Ungolianth, generally considered to be one of the Maiar corrupted by Melkor, was powerful enough to contend with the Enemy himself. Other spirits, the Balrogs, were often given command of the army of Angband and formed the core of its most formidable corps.

Other Maiar, not quite as intimidating, were given the shapes of wargs and werewolves or, according to some, even orcs like Gorbag and Shagrat. Many kept their allegiance to the Light though, and assumed the shapes of wolfhounds, Huan being the most remarkable of these, or mearas, the ancestors of the famous horses of Rohan.