I use gkrellm, and set the three memory krells option. The "usage" is memory actually used for application memory. The other krells mark how much cache and buffers are allocated. htop does something similar, green is application usage, blue is buffer usage and yellow/orange is filesystem cache (at least, as far as I know, I might be wrong)

edit: on linux. I would assume Mac and even windows have enough granularity to distinguish.