Latin texts are available online in great abundance and variety. These include very nearly the entire corpus of surviving classical
literature; an even larger, if necessarily less comprehensive, collection of texts from the late Empire, Medieval and Renaissance periods; and a representative samplying of neo-Latin from the seventeenth through twentieth centuries.
The following are links to the richest online sources of Latin texts:
The Latin Library offers a wide variety of texts, quite clean and accurate as a whole, and in a format that can readily be copied and pasted into text files. It is indexed by author's name for the classical period on the initial page, with links to other pages for early Christian, Medieval and Neo-Latin literature.
Vicifons, the Latin page of Wikisource
Documenta Catholica Omnia offers an enormous collection of texts not only in Latin but in Greek and other languages. This is an unrivalled source for works by Church fathers, on Medieval theology, philosophy and history, and of course on the Catholic Church. But it also contains a considerable amount of classical, pre-Christian literature in the 'De rebus laicorum' sections. Many texts are in .pdf format and will require a certain amount of manipulation. Please note that this site is copyrighted by the Cooperatorum Veritatis Societas. While most of the actual texts in Latin have been out of copyright for centuries, critical apparatus may be subject to copyright protections. Copyright laws in most countries permit 'fair use' copying of copywrited material for private purposes.
The IntraText Digital Libary Latin collection
Yet other texts in Latin are available online. If you would like to work on the rare text not included in the three sources listed above, a web search may lead you to what you are looking for. Wikipaedia sites on particular authors or works often contain links to online texts.