Sometimes we forget that what is common knowledge for some is completely unheard of for others. So, I suggest we revisit the question of what NXT kit supplements we most recommend---and were to find them.
My top priority would be short NXT wires, which are available from mindsensors.com. The regular wires get in the way excessively.
(While you are there, check out their new Touch Sensor Multiplexor.)
If I only owned the NXT 2.0 kit, I’d definitely purchase some gears and the rechargeable battery set. The battery pack is available from the LEGO website. Gears can be found in large quantities from LEGO Education, or in any quantity from Bricklink.com.
No matter which kit you have, more of these are desirable:
Technic Beams with Snaps
Studless Technic Beams
The links are for the Lego Education U.S. site. They can also be found on bricklink.com and, I assume, other regional/national LEGO Ed sites.
Other recommendations from y'all?
This post has been edited for accuracy by the author.