My trusty ballpoint pen.

It helps me take notes, leaves notes for other team members and is great for prodding people with if I need an update on something and they have headphones in.

Dude, no... Leave the people with headphones in alone. Send them an email if you need to ask before you forget, but unless the building is on fire, don't disturb someone who's getting shit done.

With regard to prodding it depends whether it's something critical. No use coming into work on a Monday saying "well I emailed you friday to say the API isn't responding" We're a small enough Dev team that prodding is acceptable.