How to measure memory usage for a Live ASP.NET MVC web application?
Perfmon's counters are still a good technique (and free!).
PAL (Performance Analysis of Logs), a free tool, has an ASP.NET perfmon counter template for general health (in addition to generating reports of counter log files based on thresholds).
Check out:
Commercial tools like MemProfiler, RedGate's memory profiling tool and JetBrains Profiler are all very good (and all have free trials).