I believe the true spirit of India lies in Her folk music (lok sangeet) - music that speaks volumes about our rich cultural and ethnic diversity. Due to the efforts of several Bollywood musicians and filmmakers, many folk songs have found their way into mainstream popular culture and have helped preserve their rustic charm and appeal. That said, the number of folk songs in contemporary Hindi cinema has fallen drastically. Maybe audience taste has changed over the years but folk-based songs which have come once in a while, have found widespread appreciation. I'm talking about songs like 'Genda Phool' (Delhi 6), 'Hikkanal' (Dev.D), 'Nimbooda' (Hum Dil De Chuke Sanam), 'Bhumbro' and 'Rind Posh Maal' (Mission Kashmir) and 'Udd Ja Kale Kawan' (Gadar).

These are my ten personal favourite folk-based Bollywood songs (in no particular order):

1. 'Chalat Musafir Moh Liya Re' from Teesri Kasam (1969) : Based on folk music from Bhojpuri-speaking regions of Bihar and Uttar Pradesh (Purvanchal region).
2. 'Nain Lad Jay Hain' from Ganga Jamuna (1961) : Bhojpuri folk music again. Vibrant and very expressive.
3. 'Jhumka Gira Re' from Mera Saaya (1966) : Marathi folk, “Laawni”. Please correct me if I'm wrong.
4. 'Kesariya Balam' from Dor/Lekin/Nanhe Jaisalmer : Very popular Rajasthani 'Maand' folk song. Probably the best known folk song in India. According to this source, Maand was sung in the royal courts of Rajasthan. Maand was a semi-classical form with a milieu, text and structure of its own that had an affinity with both the classical thumri and tappa forms as the popular folk music.
5. 'Chadh Gayo Papi Bichua' from Madhumati (1958) : Mix of East Indian folk music from UP, Bihar and Orissa. The song also has hints of Bihu from Assam.
6. 'Dil Ka Bhanwar' from Tere Ghar Ke Saamne (1963) : Beautiful composition by S.D. Burman based on folk music from East Bengal.
7. 'Genda Phool' from Delhi 6 (2009) : Popular Chattisgarhi folk song sung during marriages. Originally written by Gangaram Shivaray and composed by Bhulwaram Yadav.
8. 'Nimbuda' from Hum Dil De Chuke Sanam (1999) : Inspired by a Rajasthani folk song, usually sung by the Langaniyar and Manganiyar tribes of Rajasthan. Nimbu (lemon) is one of the precious fruits grown in the thorny-shrub areas of the Thar desert. Probably due to its value and importance, this folk song was created in its honour. Neem is another tree around which many folk songs have been written.
9. Nimbudo (Vocal) Uploaded by Bhargav Saikia 'Ramaiyya Vasta Vaiyya' from Shree 420 (1955) : This priceless composition was inspired by a folk song which the musicians (Shankar-Jaikishen and their team) had overheard while passing by a construction site in Mumbai (then Bombay). Ramaiyya Vasta Vaiyya in Telugu means, "Ram, when will you come?"
10. 'Bhumbro' from Mission Kashmir (2004) : Based on a very popular folk song from Kashmir.

Additional info: The tar-shehnai prelude of Aaja Nachle's title track is inspired by Angshuman Roy's Dada Paye Pori Re, a folk-based Bengali song. You can download the song here

You can also find folk music's influence on contemporary pop bands. One of the finest examples is The Raghu Dixit Project. Their self-titled debut album has songs inspired by folk music from Bengal (Baul sangeet), Rajasthan and Karnataka. I highly recommend their music. ('Gudgudiya Sedi Nodo' by Raghu Dixit)

Also check out Angaraag Papon Mahanta's 'Nanda Geet' and 'Dhonseng' (traditional Assamese Bihu).

Thank you for reading!

Acknowledgements: Nimbudo (Indian Raga), Ramaiyya Vasta Vaiya (Shikha Jhingan), Genda Phool (Musings), Photo credit (Julien Lagarde)