How to throttle bandwidth on a Linux network interface or app?

trickle should do what you want. If your running Ubuntu (or Debian, I think), you can install it with sudo apt-get install trickle, and then run it. trickle -s -d 10 -u 10 firefox (or so, I haven't used it in a while) would run firefox, limiting its download and upload speed to 10 kilobytes a second.

tc qdisc add dev tap0 root netem delay 50ms loss 50%
To add 20ms delay "tc qdisc add dev lo root handle 1:0 netem delay 20msec", and to remove it "tc qdisc del dev lo root"

wondershaper eth0 1000 200