Space Pen
A pen independently developed by inventor Paul Fisher. The Space Pen has many useful abilities: it can write on greasy surfaces, under water, and (because of its pressurized interior) in the zero-G or microgravity environment of space, using a special viscous ink that resists leakage. Much more information about the pen can be found at its Wikipedia entry.
Some time before the events of SYWTBAW, Nita is given a Space Pen of the "bullet" design by her Uncle Joel: it becomes a treasured possession, the "lucky pen" that she uses to take tests. Nita is extremely upset when it's stolen from her by Joanne Virella and her gang of bullies after they beat her up. Her attempts to get the Space Pen back become her first efforts at wizardry. These lead to her meeting Kit for the first time, and also produce some very strange results...like Fred.
Nita recovers the pen with Fred's assistance during the events of SYWTBAW. As of AWOM, she still has it, but has stopped using it for tests, possibly suspecting that -- considering where it's been and what it's gone through -- its ink may now possess properties that would be wasted on mere paper.
Fisher's own website, showing the various models, can be found here. (SYWTBAW et al.)