Q:

A good source to learn the Pali language?

I have asked this considering the importance of Pali in Buddhism. 
I want to learn Pali.
 I want to learn it individually on my own. 
Is there any relatively simple way? 
Can anyone suggest me anything?
Any free learning sites will be appreciated.

A:

(Bear in mind, I don't have mastery over the language but I've been swimming in it continuously for the last two years. The following advice is based on this experience.)
There is a whole list of books about learning Pāḷi. Some of them you can find here. I suggest applying the grammar that you learn from them, in Yuttadhammo's Digital Pali Reader (DPR) which will help you learn the vocabulary. DPR has an easy-to-use interface, dictionaries & various helpful tools including one of the grammar books mentioned in the first link.  
These are the resources I use and I complement them with the available translations on ATI, SC & in secondary literature. I also confer this Sanskrit dictionary.
I hope this information will be useful. Good luck.

A:

Courses

Partiyatti Learning Center offers two free online Pāli courses.
Pali Online School at the Oxford Centre for Buddhist Studies is a very intensive 3-week online Pāli course, that involves live instruction by prof. Richard Gombrich, a well-known scholar of Theravāda. The aim is to enable students to read original canonical texts in Pāli, with the aid of dictionares and grammars. The course is expensive, but I've taken it and can wholeheartedly recommend it. I doubt there is any other way to learn a good deal of Pāli in such a short time.

Dictionaries

Pali-English Dictionary (PED) by Rhys Davids and Stede is the most comprehensive fully published Pāli dictionary available to date. It is freely available online, but the search interface has some strange bugs (the anusvara needs to be typed as ŋ instead of the standard ṃ, and the first letter of a headword must be written in uppercase if it contains a diacritic, e.g. Āsana instead of āsana). http://dsal.uchicago.edu/dictionaries/pali/
Several Pāli dictionaries, including the PED (though not all the entries), are available here. The search interface is much more user-friendly, but lacks more advanced features of the aforementioned one (it's not possible to search for words in definitions, words that start/end/contain a particular character sequence, etc.). https://palidictionary.appspot.com/
A PDF version of the PED is freely available, as well as a text version in a single big file.
Dictionary of Pali by Margaret Cone is an improvement over the PED, with more usage examples, but only volumes I (A-Kh) and II (G-N) have been published (note the different alphabetical order in Pāli).

Books

A Pali Grammar by Wilhelm Geiger, revised by K.R. Norman

Describes phonology (the sound system) and morphology (how words are inflected), but has no information about syntax (how words are combined together)

An Introduction to Pali by A.K. Warder

generally regarded as the standard Pāli textbook
teaches about compounds (samāsa) and about some aspects of syntax and idiomatic language use, that is, the topics that are absent in Geiger's grammar
however, according to prof. Gombrich, the book cannot be relied on, because it contains strange mistakes, and sometimes is unclear or misleading

If you follow a tradition that involves chanting in Pāli (by the teacher and/or by the students), it may be useful to take a look at the chanted texts and their translation. In such case you are already familiar with the pronunciation and rhythm of the text, so learning should be much easier. For example, this book may be useful for people who follow S.N. Goenka's tradition:

Gem Set in Gold, Dhamma Chanting: English Translation with Pali and Hindi by S.N. Goenka

General

Access to Insight provides a list of resources for learning Pāli.
The Pāli Canon and its translations are obviously important learning resources, available on SuttaCentral and Access to Insight. See also answers to this question.
Here is a nice set of well-known Pāli chants on YouTube, with their transcription and translation into English. It's a good starting point for learning Pāli pronunciation.
For learning vocabulary, I recommend making flashcards in a spaced repetition program (the most popular one is Anki). Spaced repetition is a general way to memorise things in the long term, and it is tremendously useful for language learning. It is especially useful if you don't read Pāli texts regularly and don't want to forget the words.

