Hosts that negligently allow (do not implement technical measures to block) packets to be sent from an IP address not routed to the sender.
Ecatel is the big one here. I don't know what it will take for their upstreams to shut them down, but it needs to happen. Do that and many of these reflected attacks will stop.
You can send spoofed packets from nearly every host.
However hosts like Ecatel are known to specifically allow their customers to send spoofed packets at full speed 24/7.
I think most hosts will notice heavy bandwidth usage, investigate, and then terminate your account. This is why people buy servers at Ecatel even if it is more expensive.
Ecatel is the big one here. I don't know what it will take for their upstreams to shut them down, but it needs to happen. Do that and many of these reflected attacks will stop.