What were the oïl languages, and are any of them spoken today?

Oïl languages refer to those modern-day descendants considered as having evolved in their own way separately from the varieties of the ancient langue d'oïl. Consequently langues d'oïl is used to apply either to all the modern-day languages of this family except the French language, or to this family including French.

Oïl dialects is also used to refer to the Oïl languages (except French) as some modern-day Oïl languages are very close to French. However, the term dialect is sometimes considered pejorative, and since the latter half of the 20th century the tendency in French has been to refer to these languages as langues d'oïl rather than dialects.

French is now the most well-known of the Oïl languages.

https://en.wikipedia.org/wiki/Langues_d'o%C3%AFl#O.C3.AFl_languages

The French Wiki page has a better listing of the actual languages (or dialects). There also seems to be a difference between oil language (singular) and oil languages (plural).

NOTE:

Langue d'oïl -- In the singular, Langue d'oïl refers to the reciprocally intelligible linguistic variants of romana lingua spoken since the 9th century in territories now occupied by northern France and part of Belgium (Wallonia), as well as those spoken since the 10th century in the Channel Islands, and between the 11th and 14th centuries in England (Anglo-Norman language). Langue d'oïl has been used, in the singular, since as far back as the 12th century to refer to this ancient linguistic grouping as a whole. Consequently, with this meaning, in the singular, it is sometimes used to refer to Old French.

Oïl dialects -- In the plural, Oïl dialects refer to the varieties of the ancient langue d'oïl.

See BRY2K's link.

[Note added -- Zb]

To be opposed to "langues d'oc". https://en.wikipedia.org/wiki/Occitan_language
Occitan language - Wikipedia, the free encyclopedia
As far as I can remembr oïl stands for modern French "oui".

In fact, oïl and oui are essentially the same in pronunciation (as English we) - the terminal L would be easily dropped in common speech (like yes becomes yeh). Oui became the written form in Paris, and with the increase in literacy following the introduction of printing (and the predominance of Paris over the rest of France) the Paris standard became the French standard. The langues d'oc managed to survive better than the langues d'oïl because of the greater distance from Paris, and the late absorption of some regions into France.
Walloon, Lorrain and Champenois are survivals in the oïl family, and the Norman French still surviving in the Channel Islands too.
https://en.wikipedia.org/wiki/Langues_d'o%C3%AFl et al including http://www.lexilogos.com/etymologie_oil_oc.htm