Literature:

Panjab Digital Library
Punjabi Kavita (Poetry and Other Literature)
Punjabi Wikisource (Digitized and Transcribed Literature)

Grammar:

Punjabi Reference Grammar Textbook (Punjabi University 2013)
Punjabi Grammar Checker (Gurmukhi)

Vocabulary:

500 most common Punjabi words (Punjabi University)

Encyclopedia:

Punjabipedia (Punjabi University)

Learning Punjabi:

Learn Punjabi (Punjabi University)
eLearn Punjabi Course (Punjabi University)
Punjabi Bootcamp (Videos)

Language Tools:

Punjabi Morphological Analyzer (Gurmukhi)
Punjabi Part of Speech Tagger (Gurmukhi)
Punjabi Phrase Chunker (Gurmukhi)
Gurmukhi Font Converter
Punjabi Programming Tools