philolog.us - There is a great, comprehensive Latin dictionary under the ‘Lewis and Short’ tab. It has information on meanings, irregular forms, usage, etymology and examples taken from Latin authors that show how a given word is used in context
Wiktionary, the free dictionary - Wiktionary can be pretty helpful as a simpler dictionary. It is possible to search any form of a word, which helps out if you have issues identifying something. It’ll usually provide meanings, conjugations, and occasionally etymology and related words. It isn’t as in depth as phililog.us, and I have had moments where the information listed on wiktionary has been wrong, so double check with other sources if you can
Latin-English- This dictionary can be useful for giving multiple meanings and forms which are possible for a given word. This dictionary is particularly useful when identifying words and their forms while translating. There are places where there can be incorrect information listed, so try to double check with other sources.
Perseus Digital Library - This site has a large library of Latin texts. It can also show you the breakdown of a given word’s grammar, but I have found that it isn’t always totally accurate.
http://dcc.dickinson.edu/vergil-aeneid/vergil-aeneid-i-1-11 - This is a good commentary for Vergil’s Aeneid that seems to be aimed at beginners, with lots of vocabulary and grammar notes. Also includes selections from the text itself.
http://vergil.classics.upenn.edu/vergil/index.php/ - This is another site on the Aeneid that combines a couple historical commentaries and translations with a grammatical analysis of each word, including its case, number, gender, tense etc. with its syntax (i.e x is a direct object of y, a modifies b)
ET Merrill, Commentary on Catullus, Poem 1, line 1-3 - This is a commentary on the poems of Catullus, organized by poem and line number. While it seems aimed at more advanced students of Latin, it does include all the poems.
THE ALPHABET - This is a digitized version of an older grammar book, Allen and Greenough’s New Latin Grammar for Schools and Colleges. It contains extensive grammatical notes, comprising an entire Latin grammar. Useful for learning or revisiting grammatical concepts.
Bennett's New Latin Grammar - This is another older Latin grammar, in case you want an alternative to Allen and Greenough.
http://lexicity.com/language/latin/- An impressive collection of multiple sources of dictionaries, grammars, charts, and more!
Latin1: The Easy Way - This has a number of grammatical lessons suitable for beginning students presented in a casual, easy to digest style.
Latin - This site offers a number of games and exercises targeted at training specific skills. Mostly aimed at beginner students, but has a lot of variety in terms of topics to study. It mostly contains resources for practicing declining and conjugating words, but there are some options for training specific grammatical constructions as well.
Wheelock Latin Exercises - This site offers a wide variety of Latin exercises targeting specific skills and concepts. It’s basically an adapted version of a workbook that would normally accompany a textbook, so it is divided into lessons and chapters dealing with individual topics. It has material for beginning as well as intermediate students.