Automatron: March, $10
The Mechanist (who you may remember from a hilarious Fallout 3 sidequest) has unleashed a horde of robots on the Commonwealth, and it's up to you to stop him. In addition, you can harvest their parts to make your own handy robot companions, with hundreds of options to choose from!
Wasteland Workshop: April, $5
In addition to adding new options to settlement building, this add-on lets you capture creatures from the wasteland, and tame them or run your own fighting pit.
Far Harbor: May, $25
This one looks beefy. They say it's the largest add-on they've ever created, so this seems to be more in line with the Shivering Isles expansion to Oblivion (literally the greatest DLC ever released) rather than the smaller chunks we've gotten used to in modern Bethesda games. Far Harbor takes you to Maine, where Valentine's Detective Agency sends you to investigate a missing person and a synth colony. This one also includes higher level weapons and armor for you to slaughter your enemies more easily. And we can hope that there are more settlements that need our help for Preston to mark on our maps.
Bethesda also stresses that these aren't the only add-ons coming to Fallout 4; they're planning $60 worth of content in 2016.
If you're planning to get these, you should grab the season pass now. They're making more than initially planned, so on March 1, the price is going up from $30 to $50. Still a better deal than buying them all individually, but $30 is better than $50.
Want to try these out early? Sign up for the beta! https://account.bethesda.net/en/settings/beta
In addition, Bethesda is hard at work on the Creation Kit and some Survival Mode changes. They plan to continue releasing free content/mechanics updates through patches. The most recent patch added weapon debris for video cards that can handle it, added visual cues to help determine where settlers are assigned, and more, along with fixing some bugs and making some performance improvements.
Lots to look forward to!
For the most recent patch notes: http://fallout.wikia.com/wiki/Fallout_4_patch_1.3