Oink - Oink is a Rails plugin and log parser to help narrow down the source(s) of increased memory usage in rails applications.
strace - Trace system calls and signals. A diagnostic, debugging and instructional userspace utility.
Munin - PnP networked resource monitoring tool that can help to answer the what just happened to kill our performance
Valgrind - Valgrind is an instrumentation framework for building dynamic analysis tools.
Derailed Benchmarks - Derailed Benchmarks is a benchmarking tool for Rails.
perf - Perf is a simple app monitoring solution paired with meaningful alerts.