Parallel Pliers
Parallel Pliers are not often seen on building sites or diy jobs, despite their utility. Parallel Pliers give much greater grip on nuts and other flat objects than conventional pliers or waterpump pliers. Nuts that were simply impossible to undo with large standard pliers were quickly and easily undone with smaller parallel pliers.
The downside of small sales volume is that most tool outlets don't stock them at all, and when found they're not cheap. However if you need a tool that will undo all sorts of things with ease, they're a good investment.