Not an exhaustive list, just some sites I’ve found especially useful studying and teaching. Scroll down for fancier Classics Masterposts. Updated August 26, 2015.

Latin

words

http://latinlexicon.org/ (my favorite; dictionaries (L&S) and flashcards)

http://archives.nd.edu/words.html (if I can’t convert you to latinlexicon)

http://nodictionaries.com/ (neat concept, sometimes misleading)

http://romansgohome.com/spqr (if you have the right devices…I don’t) 

http://www.wyomingcatholiccollege.com/faculty-pages/patrick-owens/lexicon/adumbratio/index.aspx

texts

http://www.thelatinlibrary.com/

http://latin.packhum.org/index

http://www.stoa.org/colloquia/anglice.shtml

http://vergil.classics.upenn.edu/

other fun stuff

http://bestlatin.blogspot.com/

https://la.wiktionary.org

Latin AND Greek

http://dcc.dickinson.edu/ & Latin core vocab & Greek core vocab (learn these)

http://www.perseus.tufts.edu/hopper/

http://perseus.uchicago.edu/

http://www.textkit.com/

http://linguax.com/lexica/

http://attalus.org/

http://lexicity.com/ (many other languages too)

Greek

http://www.greekverbs.com/ (identifies and provides the paradigm!)

http://www.typegreek.com/

http://www.lib.uchicago.edu/efts/Woodhouse/

Culture etc. (obviously this is not comprehensive)

http://www.theoi.com/

https://www.beazley.ox.ac.uk/index.htm

http://digitalaugustanrome.org/

http://penelope.uchicago.edu/Thayer/E/Roman/home.html

https://medium.com/eidolon

Teaching Latin (mostly CI) (good stuff for learners, too)

https://latinbestpracticescir.wordpress.com/

http://indwellinglanguage.com/ (Latin media)

http://todallycomprehensiblelatin.blogspot.com/

http://latinforeveryone.blogspot.com/

http://pomegranatebeginnings.blogspot.com/

http://www.johnpiazza.net/latin

http://www.latinahilara.com/

http://lapis.practomime.com/index.php/698 (embedded readings for AP)

http://www.practomime.com/content/verba.php (like Cards Against Humanity but in Latin and classroom-safe, though you can be creative)

http://www.practomime.com/picturae/

http://www.tcl.camws.org/ Teaching Classical Languages

http://www.ijflt.com/ Foreign Language Teaching

Other Lists

c-aesarion​‘s Comprehensive Online Resource List for Students of Classics (!!!)

aristoteliancomplacency​‘s Master Post of Classicists on Tumblr. 

clyternnestra​‘s Ancient Greek Masterpost