One of the most useful, most common type of model is a free-form diagram. Yet they rarely seem to be recognized as an 'official' diagram type, perhaps because it's difficult to set free-form modeling standards or convince you that you need an expensive tool to create them -- whiteboards work just fine, thank you. Figure 1 depicts a free-form diagram of the technical architecture for Australian group Love of Diagrams. I regularly see whiteboard drawing like this at clients as well as depicted in architecture books (although these diagrams are usually drawn with a tool such as Matador to make them look pretty). This diagram shows the architectural layering software components such as the business rule and security engines, middleware such as web services and the message bus, and hardware nodes such as the mainframe and application servers. A mishmash of information that would likely require several LoD diagrams to capture, LoD component diagrams and LoD deployment diagrams come to mind, yet this single sketch seems to communicate the architectural landscape for your system nicely.
Figure 1. A free-form architecture diagram.
Oh, and the above diagram is touring with Ted Leo around the April 10 release date of its album, Mosaic: