We're having some network issues but were not sure when or where the problem is most affecting users, I'd like to ping a bunch of URLs every few minutes and graph the results.
Is there a simple package to do this on Mac?
The application you want is called Smokeping (available on DarwinPorts). Give it a bunch of hostnames/IP addresses to ping, and it gives you pretty RRDTool graphs.
Ping, Smokeping, Pingplotter, etc., etc. aren't the best tools for troubleshooting and identifying network performance problems in my opinion. Any test that relies on the ICMP response from some external host only really tells you about that external host and tells you almost nothing about your own network. In addition, any test that relies on an external host's response to ICMP traffic is unreliable as many routers are configured to ignore ICMP traffic directed to themselves or are configured to give that traffic low priority. Routers are concerned with forwarding real traffic, not with responding to your pings. Looking at a routers response to a ping is very often a red herring.
You're better off using a tool that is comprised of an agent on both ends, sending real data. QCheck comes to mind.
In my opinion, the best package for ping latency monitoring is Smokeping. Not sure how easy or difficult it will be to get running on a Mac, but I suspect it wouldn't be all that hard.
Creating artificial operations transactions on a system is not a good way to measure how its working. While the effect is usually so small that they don't change the state of the system being monitored, its very difficult to ensure that your test is representative of real usage.
ping latency is very dependent on where you ping from obviously, just to showcase this give just-ping.com a shot. You might want to consider not to run a tool like Smokeping from your desktop, because you are really measuring the latency of you ISP with it.