While johndunn1970 is freaking amazing, and I do like mharvey visio diagrams too (and used similar in the past), I find them both to be harder to maintain if you end up swapping gear around. I went with a really simple "rack" layout. I basically coloured some cells in Excel, took a screen shot, saved it as a JPG and then used it as a background in Atlas, and drew simple squares for the servers to match the U positions. You can see it at work in this thread. These tend to be easier to maintain, if you pull a server, you just delete the shape. Add a server, draw another shape. I do love the above two examples, if I had enough time to maintain, I'd probably do the same.
Edit: I forgot you actually mentioned application monitors. If there are specific applications we wanted to represent on the rack diagrams as well, I'd throw a circle shape with status next to the server. The rack will change color in the overall datacenter view shown in the previous thread. There are very few we do that for at the rack level though, as we have application level maps that break out servers by tier.
