Dead Nation Apocalypse Edition (PS4)
One of my favourite PS3 games, so would have bought it again anyway. Having it free was nice. Mapping fast weapon/item selection on the face buttons was a massive improvement, and this alone makes this the definitive version of an incredible game. The prettier 1080p graphics are just a bonus.
I buy a ton of PSN games, so many times when older games come on PS+, chances are I've already played them. There are still a bunch of awesome games that were either ports of games I'd already played, on games I'd not wanted enough to purchase first time around.
PixelJunk Shooter Ultimate (Vita)
Replayed both games on the Vita. Really good fit for the handheld.
Puppeteer (PS3)
If this hadn't been free, I wouldn't have played it. Such a sumptuous game. The production values are staggering.
Proteus (Vita)
Likewise, if it hadn't been on PS+, it would have passed me by. Not much meat on the bone, but I still spent a few train commutes transfixed by its lo-fi virtual island. My hype for No Man's Sky comes mostly from thinking it looks like a big budget Proteus.