I feel foolish for having made this error (especially after having so patiently explained how this stuff works on Mach), but today I did it. I reported the amount of free memory on a Linux system as being the amount reported by free as free.
My own opinion on this is that I suffer from a view of hardware management which was tainted by using micros like the Dragon 32 (Radio Shack/Tandy TRS-80 to my American readers) and the Amiga, where there were basically two types of memory usage: yes and no. A particular block was either in use by the system, or it wasn't. On the Dragon 32 it was even easier than that of course; all bytes were available for reading and writing, but what happened was context-dependent. Also because this was the MC6809E, whether such a peek/poke made sense depended on whether you were trying to hit an I/O address, and what was plugged in. The Amiga had a particularly lame memory allocator which quickly sucked performance like a vacuum of performance suckage +2; but a byte of RAM was either in use or it was available.
But I digress. The point is, that such a simple view of memory availability is no longer sensible but it's hard for me to think around it without a lot of work, just as it was hard for me to become a programmer after I'd been taught BASIC and Pascal. If I were involved in UNIX internals more (and indeed that would be fun, although I think maybe Linux wouldn't be my first choice to open up) I'd probably be able to think about these things properly, just as I had to throw myself into C programming in a big way before I lost my BASIC-isms.
For the record (and so that it looks like this post is going somewhere), both operating systems have an intermediate state for RAM to be in between "used" and "not used" (where I'm ignoring kernel wired memory, and Linux kernel buffers). On Mach, there's the "inactive" state which I've already described at el linko above. On Linux, it's used as an I/O cache for (mainly disk, mainly read-ahead) operations. This means that Linux will automatically take almost all (if not all) of the memory during the boot process. The way that inactive memory gets populated on Mach means that on that system (e.g. Darwin) actually the amount of free memory starts large and inactive starts small, but over time as the active and inactive counts go up, the free count goes down, and it's rare for memory to be re-freed. On both systems, free memory is really "memory it wasn't worth you buying" as it's not being put to any use at all.