How to simulate slow internet connections on the mac
Apple’s official tool to slow down the network connections on you Mac for testing purposes is Network Link Conditioner

Additional Tools for Xcode [version].

Additionally, iOS has similar function accessible from within Xcode and iOS 6 or later.

Older versions of Xcode before version 4.3.2 embedded a copy of this tool. This SO thread documents some history of the tool 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 create custom profiles with your own settings or 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.