I'm no expert, but here's some stuff I'm familiar with. (It's not my intention to embed ALL the videos, by the way... otherwise this would get too crazy, and it's not all 100% on-topic

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Modern "Ambient":
Chillwave is the more popular offshoot; it's been discussed in plenty of places on this forum already. Not ambient, but certainly derived from a combination of ambient, hip hop, pop, shoegaze etc.
Memory Tapes, Toro Y Moi, Com Truise, Miracle Fortress, Neon Indian, Namine, Beat Connection, Chilly Willy, Blackbird Blackbird, Teen Daze were recommended by boardies in response to this. I'd add Pictureplane, Baths, Teengirl Fantasy, Star Slinger.
Neon Indian "Deadbeat Summer"
http://www.youtube.com/watch?v=qlJ27Dcv4fc
Washed Out - "Feel it all around"
http://www.youtube.com/watch?v=-DkslcOhytU
Baths - "Lovely Bloodflow"
http://www.youtube.com/watch?v=tVTrZjY2K5I
http://www.brainfeedersite.com/ - label incorporating elements of hip hop and dubstep with a strong ambient feel. Bassier and faster than old-school ambient in many cases.
Matthewdavid