I recently read that Kubera is exemplified by Eight Treasures or Ashta-nidhi. What are they?
Yes, Kubera is exemplified by eight treasures. There is a mention of it in the Śrīmad-Bhāgavatam tenth skandha shloka 55.
śyāmaika-varṇān varuṇo hayān śuklān mano-javān
aṣṭau nidhi-patiḥ kośān loka-pālo nijodayān
Lord Varuṇa offered horses as swift as the mind, some of which were pure dark-blue, others white. The treasurer of the demigods, Kubera, gave his eight mystic treasures, and the rulers of various planets each presented their own opulences. [10.50.55]
Sridhara swami while commenting on this verse gives the name of these treasures.
padmaś caiva mahāpadmo matsya-kūrmau tathaudakaḥ
nīlo mukundaḥ śaṅkhaś ca nidhayo ’ṣṭau prakīrtitāḥ
The eight mystic treasures are called Padma, Mahāpadma, Matsya, Kūrma, Audaka, Nīla, Mukunda and Śaṅkha.
So, the eight treasures are:
- Audaka (Makara)
- Mukunda and
The details of some of these Nidhis is given in Vamana Purana chapter 76.
Imperishable Nidhis are existed in their complexion. History, PurAna, Vedas with their parts (Angas), Veda, Smrtis, sixty four Kalas and Mahapadma Nidhi are vested with the goddess having white complexion. Pearls, gold, silver, chariot, horse, elephant, garments, weapons, arms and Padmanidhi in the form of garments are vested with the goddess having red complexion. Cow, buffalo, donkey, camel, gold, garments, land, medicines and MahAnila Nidhi in the form of animal are vested with the goddess having yellow complexion. Śañkha Nidhi in the form of the best among all castes embedding with her all other castes is existed with the goddess having blue complexion. O demon! Now I am going to describe the characteristics of Purusas falling under these complexions of Nidhis. Understand these all. [31-35]
O demon king, The people sheltered to Mahapadma observe truth and purity and always engaged with performing yajña, donation and festivals. 
The man with Padma perform yajña, enjoy luck, egoist, honour preferring, generous to high level and enjoy more pleasure as compared to the common people. 
The person vesting MahAnila are found using truth and false in mixed form, clever while doing transactions. They do justice, injustice and extravagant. 
O Bali! The people vesting Śankha are found atheist, unclean, miser, away from luxury, thief and liar. O demon! I have thus, described their complexion to you. 
So, the complexions of these Nidhis are:
Mahapadma Nidhi - White.
Padma Nidhi- Red.
Mahanila Nidhi - Yellow.
Śaṅkha Nidhi - Blue.
The names of these treasures are also given in a separate chapter (chapter 53) in Garuda Purana Karma Kanda. The names are slightly varied.
After hearing from Hari, Brahmä described the eight Nidhis. They are Padma (Lotus) Maha padma, Makara, (Crocodile), Kachhapa, (Tortoise), Mukunda, Nanda and Śaṅkha. These Nidhis are conducive to the flourishing of qualities Sattva, etc. I shall now describe their special characteristics. [1-2]
The two nidhis Padma and Mahapadma are Sattvika in characteristics. A person having the marks of makara gathers together swords, arrows, javelins, etc. 
Makara and Kacchapa — these two nidhis are supposed to be tamasika in characteristics. A person having the marks of Kacchapa does not have faith in any person. He neither enjoys his wealth nor gives it to anybody. 
He will keep his wealth boarded in a deep pit in the ground. He will remain a single person (without any friend). The nidhi Mukunda is rajasika in characteristics. A person having the marks thereof collects realms. 
He enjoys all good things in life and distributes wealth among musician and courtesans. A person having the marks of Nila - both Tamasik and rajasika and shall be the vital support to his family. 
A person having the marks of the nidhi Nanda shall have :sattvika splendour. He will gather together garments, grains, etc: 
He has the manliness and power of three persons. He will be making mango groves, tanks, etc. The nidhi Śaṅkha indicates self-centredness. He spends all his wealth in his own enjoyment. 
His servants and kinsmen eat wretched food and do not wear good garments. The Śaṅkha (one bearing the marks of Śaṅkha) is assiduous in nourishing himself. If, ever he gives anything to anybody it shall go in vain. 
When the markings of different nidhis are present the results are mixed. Lord Hari had thus expounded to Hara and others the characteristics of the nidhis. I am expounding now as Hari had expounded them previously. 
Characteristics of the treasures:
Sattva : Padma, Mahapadma, Nanda
Rajas : Mukunda, Nila (it is both Rajasik and Tamasik)
Tamas : Śaṅkha, Kacchhapa (also known as Kurma), Makara (Audaka)