This can be done with native osx tools as well. Here's a snippet I created once:
# First add a rule for all local traffic to port 80 to go into pipe 1
# 100 is the rule number which will be used for referencing the rule later
sudo ipfw add 100 pipe 1 ip from 127.0.0.1 to 127.0.0.1 dst-port http
# To display the rule use
# sudo ipfw show 100
# configure the settings of the pipe as you please
# 50kbit/s bandwidth
sudo ipfw pipe 1 config bw 50Kbit
# 200ms lag
sudo ipfw pipe 1 config delay 200ms
# 20% random packet loss
sudo ipfw pipe 1 config plr 0.2
# To display the current connections on the pipe use:
# sudo ipfw pipe 1 show
# when finished testing don't forget to delete rule and pipe
sudo ipfw delete 100
sudo ipfw pipe 1 delete
