Latin Library at Packard Humanities Institute - http://latin.packhum.org/ (PHI numbers standard way to refer to Latin texts, look at the ones Perseus uses - it’s PHI).
Brepolis - http://www.brepolis.net/ - may need to access this via your institution or its ezproxy (includes the Library of Latin Texts A and B LLT-A and LLT-B and many other interesting resources).
L’Annee Philologique - http://www.annee-philologique.com - another one in which you’ll have to use via your institution’s ezproxy or other online database (we use ebscohost). Many journals you submit articles to expect references to other journals use the abbreviations in APh.
For databases of journals, first start at JSTOR - http://www.jstor.org - again, institutional access is required.
The Latin Library - A collection of Classical and Medieval texts in Latin, organized by author.
The Internet Ancient History Sourcebook - A collection of mostly primary source texts translated into English. Not comprehensive, but covers a broad range of topics.