The great joy of the internet is how many ancient sources are now available online, so one doesn't have to run to the library to check every single reference ... It also means that I managed to have a bit of a clear out of books.
Perseus - I personally it not particularly user friendly, but some people love it and it is in theory the 'best' site for Greek and Roman literary sources ...
Lacus Curtius is the site I love best - Billy Thayer has added the Loeb versions of many of the major Greek and Roman texts.
Attalus has a very useful list of many of the ancient sources.
The Latin Library is the place for Latin texts (without English translations).
Corpus Scriptorum Latinorum has many sources in both in the original and in translation - it tends to be the place I find things that I never thought I would ...
Roger Pearse's Early Church Fathers gathers many of the early Christian sources.
Remacle has gathered some of the sources relating to Marius' life until his exile; in the original with French translations.
Pseudo-Aurelius Victor, viri ill / Les Hommes Illustres de la Ville de Rome - in French.
Asconius - various in Latin.
Paulus Orosius, Historiae Adversum Paganos - in Latin.
PACE has both Polybius' Histories in Greek and in English, and Walbank's Commentary here (the page also have Josephus).
And for academic articles ...
I know that many of us get frustrated with JSTOR access rules, so I wanted to draw people's attention to Persee, the French version - it's free, and easy to use.