Anybody knows that Panduranga. Is he a devotee of Lord Vishnu or an avatar of Lord Vishnu or else a normal human sage who has birth and death?
Yes, he is believed to be an incarnation of Vishnu, more precisely the Krishna himself. You can take a look at Legend of Pundalik to know some history. He is primarily worshiped in Maharashtra Pandhapapura. Very useful information found in this Hindupedia article. Actually I wanted to quote some text from books "The Vithoba of Pandjarpur from Advait Ashrama but it seems url has changed.
Quoting from Vitthal Rukmani Mandir's official site:
The parabrahma or the God of Pandharpur is worshipped and lovingly called by his devotees with many names in different course of the time, like Pandharinath, Pandurang, Pandhariraya, Vithai, Vithoba, Vithumauli, Vitthal gururao, Pandurang, Hari etc. However, today this God is well-known as Pandurang and Shri Vitthal. Many historians and researchers tried to find out the etymological origin of the word “Vitthal”. Some scholars believe that it is a distorted form of the original word Vishnu. The words like Vittharas, Vitta found in various Kannad epigraphs are basically the elaboration of the word Vishnu. The Great Saint poet Tukaram defines the word Vithoba in one of his abhangas that stands for ‘Knowledge’ + Thoba Stands for ‘form’ Thus Vithoba stands for the ‘form of ultimate Knowledge’ or ‘idol of ultimate Knowledge’. It is also believed that Vi stands for bird Eagle + Thoba Stands for sitting place, thus Vithoba stands for the ‘God who sits on Eagle’. Vithoba is God Vishnu, standing on a brick and resting his arms on his west. It is believed that Shri Krishna, Shri Vishnu and Shri Vithoba are all different names and forms of the one and the same God. Shri Krishna is known as incarnation of Shri Vishnu which took place on Wednesday (Shravan Vadya Ashtami) at the end of Dwaparyuga. Vithoba is Shri Krishna only. Wednesday is known as the day of Vithoba. So devotees (varkari) of Vithoba never leave Pandharpur on Wednesday even now.
There is a verse in Purana, the holy scripture of the Vedic religion
Vi karo vidhatay, tha karo nilakanthay |
La karo lakshmikant, vitthalabhidhineeyame ||
ttha-nilakantha God Shankar,
thus it leads to say that all three deities Brahma, Vishnu and Mahesh are signified by one name Vitthal or are included in one God Vitthal.
Related Samradaya Vanakari is founded by Pundalika and followed by Jnaneshwar, Namdev and Tukaram respectively.
It would be more useful if one add answer with citing some scriptural references.
By the way, the famous Stotra Pandurangashtakam composed by Adi Shankaracharya also mention this legend:
महायोगपीठे तटे भीमरथ्या वरं पुण्डरीकाय दातुं मुनीन्द्रैः ।
समागत्य निष्ठन्तमानंदकंदं परब्रह्मलिङ्गं भजे पाण्डुरङ्गम् ॥ १॥
I worship that Panduranga, who is the absolute Brahman, Who is the source of immense happiness, Who stays in the great seat of yoga, in the banks of Bheema, Along with great seers to fulfill the boon to Pundarika.