There are several free tools for monitoring availability of multiple computers. However, it is surprisingly difficult to find a tool with the source code available. (For other needs, please look the discussion forums of your preferred programming language, there are several code examples available)
One good portable alternative is PingInfoView (not open source).
simple GUI, easy to use
can ping multiple hosts
can extract results
some people might like to have a graph
In the figure above, gateway, dns server etc. are selected in order to identify from where the problems arises. In my case, one computer seem to have latency peaks already in the home network, ie. the problem is not related to ISP. Other tools are needed to investigate further the problem within home network. (Update: After this tool I used a bandwidth manager to limit the traffic, but the problem persisted. Finally I used NetBalancer and then I found out that similar problems has been reported in relation with firefox.)