No spoilers: Leak contains GoT info, unaired episodes of other shows, and internal docs.
HBO has been working hard to clean up the leaked files via a flurry of DMCA takedown notices—but conversely, one DMCA notice sent to Google, and apparently seen by Variety, reveals that "thousands" of internal company documents were obtained and leaked by the hackers, including personally identifiable data and passwords from at least one senior HBO exec.
Despite the takedowns, a short script, summary, and homemade video storyboard for next week's Game of Thrones episode, "Spoils of War," turns up easily through a little Googling. The short script (which is more of an outline) is dated April 2016, and so some details may have changed. (Note: These materials definitely contain potential spoilers, though, so consider yourself warned if you really can't wait until Sunday/Monday to watch the full episode.)
The hackers claim that they extracted a full 1.5 terabytes of data from HBO. The leak appears to contain full episodes of Ballers (season 3, episodes 1-3), Barry (season 1, episodes 1-2), Room 104 (season 1, episodes 2-3), and Insecure (season 2, episode 2). Some of these episodes have already been broadcast, but some are brand new (Barry doesn't officially air until 2018, for example).
For its part, HBO acknowledged on Monday that it was the victim of a hack, but the company stopped short of confirming how much or what types of data had been obtained. In a memo distributed to HBO employees on Monday, CEO Richard Plepler said senior executives, working with outside security experts, are "working round the clock to protect our collective interests."