Internet Grammar of English--a free online course in English grammar written primarily for university undergraduates.

Purdue University's Online Writing Lab (OWL)--offers definitions of grammar and help with writing compositions in English.

Using English  --A large collection of English as a Second Language (ESL) tools & resources for students, teachers, learners and academics, covering the full spectrum of ESL, EFL, ESOL, and EAP subject areas.

Verbix - Verb Conjugator

Purdue OWL: ESL Instructor and Student Handouts -
links to pages that explain grammar and mechanics, workplace writing, etc.

Chabot College's English Verb Tenses Guide is an informal, content-packed guide on English verb tenses.