(with an eye on the New Testament in particular, last updated 4th October 2021, more updates planned soon)
The Open University have a good introduction to Greek learning.
The Eton Greek Project is helpful for practising particular word parts, although it is Classical Greek and not Koine. (N.B. this is a new [and temporary] home for the site).
If you want to use a Unicode Greek converter in order to avoid Greek fonts, try this.
Texts and translations
For the United Bible Society 5th edition text, see Academic Bible. This also has the NA28.
You can access the Greek New Testament through Perseus. The text, however, is Brooke Foss Westcott, Fenton John Anthony Hort, so not one most people will be using for academic study.
The text is available on Thesaurus Linguae Graecae, but to make full use of this, you need a login through an institutional affiliation. Watch this space for some videos on how to use TLG, with a New Testament focus. I have made a 16 minute video walking through the range of tools.
General New Testament information and study tools:
New Testament software
Logos – this is now up to version 9.
New Testament Textual Criticism
Centre for the Study of New Testament Manuscripts: http://www.csntm.org/ (Links to an external site.)
International New Testament Greek Project: http://www.igntp.org/ (Links to an external site.)
and their YouTube (Links to an external site.) channel
New Testament Virtual Manuscript Room: http://ntvmr.uni-muenster.de/manuscript-workspace (Links to an external site.)
Codex Sinaiticus: Codex Sinaiticus – Home (codex-sinaiticus.net)