Apple has made a very handy official tool to slow down the network on the mac for testing.
It's called the Network Link Conditioner and it's a free download of Xcode for Lion users. Most older versions of Xcode before version 4.3.2 embedded a copy of this tool. Now, you use Xcode to download it directly in a similar manner to the iOS simulators and developer documentation.
There are 11 built in profiles from a Lossy Edge network with 400ms delay to a cable modem. If you need other limits, you can also use
ipfw yourself as described in Craig Hockenberry's article slow ride, make it easy It also mentions the Speed Limit panel by Mike Schrag that is a smaller download than Xcode, but has fewer options than Apple's tool.
It slows down the entire network stack, so you can't throttle on a per app basis without doing things like install lion in a virtual machine and set that VM with a throttled stack.