"It's a Remembrall! ...Gran knows I forget things -- this tells you if there's something you've forgotten to do. Look, you hold it tight like this and if it turns red -- oh..."
-- Neville Longbottom (PS9)
A Remembrall is a glass ball the size of a large marble. In its default state (when nobody is touching it), a Remembrall is full of white smoke. When picked up and held in a person’s hand, a Remembrall will glow scarlet if there’s something the person has forgotten to do.
- Neville's Gran sent him a Remembrall by owl post during his first term at Hogwarts; Draco's theft of this as a practical joke led directly to Harry being selected as Seeker for the Gryffindor House team (PS9).
- Remembralls are banned from the examination hall at Hogwarts (OP31).
In his fifth year, Neville said he could do with another remembrall as he had lost the one from his first year (OP10).
The name "Remembrall" = "remember all", and also suggests the object's ball shape.
The Remembrall was also part of a question in the WOMBATs Jo posted on her website. There will very likely be few peope who would choose a Remembrall to be indispensable in case of trouble! (JKR-W2)