If you're on FreeBSD you can use dummynet to simulate packet delay and loss.
It's quite handy. My partner has slow, unreliable, high-latency Internet in his house and there are an entire class of performance problems that are extremely obvious when I work from his place that are barely measurable when I work from a 100mb line that's only a few milliseconds away from a major datacenter.
I second Sloppy. I've found it very useful for debugging Flash errors that only happen occasionally. By slowing the loading sometimes them become reproducible.
It's also useful to see how a webpage loads on a slow connection - for example: is your copy legible before your background image has loaded?
It's quite handy. My partner has slow, unreliable, high-latency Internet in his house and there are an entire class of performance problems that are extremely obvious when I work from his place that are barely measurable when I work from a 100mb line that's only a few milliseconds away from a major datacenter.