Well only sort of FREE. Something called the HBO Collection is coming to Amazon Prime on May 21st. If you have Amazon Prime then you get it for FREE. If you don’t, then you can always sign up for a FREE month and spend that month watching classic HBO.
Full details are not yet available but it appears as though it will be the full series of The Sopranos and at least the first seasons of True Blood, Boardwalk Empire, Deadwood, and The Wire. I would expect Deadwood and The Wire to be the full runs as they have been off air for quite some time and are only three and five seasons respectively.
No word yet on other series like Rome, Treme, Oz, Six Feet Under, etc. nor miniseries like Band of Brothers, The Pacific, Mildred Pierce, and others.