DelayedAckHack is a free installer which will place a property list in /Library/LaunchDaemons/ that will cause the following command to be run with root privileges every time you login:
sysctl -w net.inet.tcp.delayed_ack=0
Using DelayedAckHack is reported to ease symptoms of abysmal AFP performance many users are experiencing in Mac OS X 10.5.2, but it doesn`t seem to work for everybody.
WARNING: There are no warranties or guarantees whatsoever; use DelayedAckHack entirely at your own risk.