How to monitor the memory usage of Node.js?

The built-in process module has a method memoryUsage that offers insight in the memory usage of the current Node.js process. Here is an example from in Node v0.12.2 on a 64-bit system:

$ node --expose-gc
> process.memoryUsage();  // Initial usage
{ rss: 19853312, heapTotal: 9751808, heapUsed: 4535648 }
> gc();                   // Force a GC for the baseline.
undefined
> process.memoryUsage();  // Baseline memory usage.
{ rss: 22269952, heapTotal: 11803648, heapUsed: 4530208 }
> var a = new Array(1e7); // Allocate memory for 10m items in an array
undefined
> process.memoryUsage();  // Memory after allocating so many items
{ rss: 102535168, heapTotal: 91823104, heapUsed: 85246576 }
> a = null;               // Allow the array to be garbage-collected
null
> gc();                   // Force GC (requires node --expose-gc)
undefined
> process.memoryUsage();  // Memory usage after GC
{ rss: 23293952, heapTotal: 11803648, heapUsed: 4528072 }
> process.memoryUsage();  // Memory usage after idling
{ rss: 23293952, heapTotal: 11803648, heapUsed: 4753376 }

In this simple example, you can see that allocating an array of 10M elements consumers approximately 80MB (take a look at heapUsed).
If you look at V8's source code (Array::New, Heap::AllocateRawFixedArray, FixedArray::SizeFor), then you'll see that the memory used by an array is a fixed value plus the length multiplied by the size of a pointer. The latter is 8 bytes on a 64-bit system, which confirms that observed memory difference of 8 x 10 = 80MB makes sense.