SLA monitoring and reporting to customers

What open-source or low-budget tools are operators using for SLA monitoring when the reports (current state and historical) should be available to customers ? Looking at NANOG archives, NAGIOS is the most prevalent tool, but its authorization mechanisms are somewhat below I would like so customers could not change anything both in configuration and in SLA software state. I'm looking for something more like Cacti, where customers can be contained to only see some of the generated graphs. Thanks for any input, Rubens