Throttle

Merely proxies connections to a single server, and throttles the connection to a given speed in KB/s. This is useful for testing web sites and other network based applications for which latency and connection speed can affect the functioning or presentation of the program to the user.

I don't go to any lengths to ensure the speed is accurate, nor that asking for 500KB/s will get you 500KB/s, but I do guarantee if you ask to throttle it at X KB/s then you will not get faster than X KB/s.

Here is the --help output:

$ throttle --help
Throttle v1.0, (C) Chris Done 2010

throttle [OPTIONS]
  Listens on port <listen> and proxies a throttled connection to <host> on
  <port> at speed <speed>KB/s.

Common flags:
  -l --listen=INT 
  -h --host=ITEM  
  -p --port=INT   
  -s --speed[=NUM]  Speed in KB/s, e.g. 1.6.
  -? --help         Display help message
  -V --version      Print version information