Good news if you're doing routing simulations with OMNeT++: in the next release, Tkenv will be able to lay out the network graphics automatically, so it'll be a lot easier to follow the route of packets. You'll also be able to colorize module icons at runtime (via display string changes) to visualize node state or just to improve the look of the model. Read on for screenshots.
Automatic layouting: all modules that don't have explicit coordnates in the "p=" tag will be arranged automatically in the graphics so that they don't overlap, connected ones are close to one another, etc. You can also mix it with explicit positioning of some nodes and with specifying row, matrix, ring, etc layout for some module vectors -- the "clusters" laid out as ring,etc will be moved as one. The algorithm is based on attraction and repulsion among nodes (a.k.a Spring Embedder).
Icon colorization: the "i=" display string tag has been extended -- it supports things like "i=server1,gold" or "i=pc,cyan". The degree of colorization in percentages is also a parameter: "i=pc,red,70" is the "pc" icon in 70% red (default is 20%). It can also be used to brighten, darken or grayscale an icon. This feature can be useful to encode status info in the icon (e.g. red=overloaded, grey=down, green=idle, etc.), to group modules (e.g. all web servers are yellow) or just to make the graphics look better.