The following list of online sources of texts in ancient Greek provides access to quite an extensive collection. There are other collections as well. In addition, if there is a particular text you are seeking, an internet search or the links in Wikipaedia articles may lead you to an online source of the text you are seeking.
To view Greek texts on your browser, be sure that 'character encoding' is set to 'UTF-8' (With Firefox, go to 'View', then to 'character encoding.' With Intenet Explorer, go to 'Page' and then 'encoding.')
Contains quite a wide range of texts in ancient as well as modern Greek, listed alphabetically by author and then, on the author's page, by work. The format is readily copied and pasted into word processors, text editors and the like. This is a quite accessible and easy to use source.
A fairly extensive list of texts in a format that can readily be copied and pasted.
An as yet somewhat limited but growing list of ancient Greek texts as documents in zip format which, when downloaded can be opened directly in a word processor – an excellently convenient format for those who would like to download an entre text.
The Thesaurus Linguae Graecae (TLG) project of the University of California (Irvine) offers the Abridged Online TLG for public use. (The full version of the TLG is available only to subscribers.) The abridged version offers a quite extensive collection of ancient Greek t exts. Texts are displayed page by page only, which may or may not be a problem, depending on your needs. To view the texts in Greek, you will probabaly want to select 'Unicode' from the drop-down menu in the upper left.
The Tufts University Perseus Collection Greek texts offers a wide variety of texts. Texts are easily copied and pasted but are presented section by section only, essentially paragraph by paragraph. Depending on your needs, this may or may not be a problem.
In addition to the works of Church fathers who wrote in Greek (follow the menu to Migne's Patrologia Graeca), this site contains a substantial collection of classical Greek works. Follow the menu to 'De Rebus Laicorum', then to 'De Antiqua Aetate,' thence to 'De Philosophia,' 'De Historia,' or 'De Humanitate.' ('De Tractatis' does not include Greek texts per se), and then scroll down to the Greek section (language code GR). Formats vary. Most texts are in .pdf. Some are in .doc or .docx. Copying from .pdf formatted documents can be problematic, depending on your .pdf reader.