I’ve recently got into learning some of the more hidden functions in JOSM, and I’ve discovered a lot of cool dialogues and functions that I didn’t know existed before. Here are some of my favourite keyboard shortcuts (make sure you’ve toggled expert mode on):

• ctrl + shift + o: opens the “Download object” dialogue, allowing you to download by ID
• A whilst in node drawing mode: toggles accurate drawing mode, allowing you to align to common angles and with previous points. You can also hold ctrl and hover over another way to draw parallel with it
• ctrl + alt + v: paste copied object into the source position (i.e. where it was when you copied it). Useful when working with multiple layers
• 5: zoom to downloaded area
• 4: zoom to conflict
• ctrl + h: display history
• holding ctrl while drawing a way starts a new way from a point
• shift + j: join overlapping areas
• alt + shift + f: opens the filter dialogue allowing you to hide certain objects

I’m sure there’s many more that I’ve missed!