A software licence that's libre but not gratis?

duncan bayne writes: "My company is developing some software using Ruby. It's proprietary software — decidedly not free-as-in-beer — but I don't want to tie my customers down with the usual prohibitions on reverse engineering, modification, etc. After all, they're licensing the product from us, so I can't see why they can't use it as they see fit. Does anyone know of an existing licence that could be used in this case? Something that delivers the customer the freedom to modify the product as they see fit, but prohibits them from creating derivative works, or redistributing it in any fashion?"