Decanter of Endless Water (9,000 gp) The Decanter has a thousand uses beyond just providing drinking water. Put out fires with it. Use it as a jet propulsion system for a small boat. Create sophisticated hydraulic technologies using it as a power source. Combine it with Water Walking to produce an instant bridge. Cool off hotheads at taverns. Endless usage :P
For all those sneaky characters out there the Collar of Umbral Metamorphosis from Tome of Magic is definitely a must. For 10,800gp you can gain the Dark creature template for 10 min a day.
In addition to such goodies as +8 Hide, +6 Move Silently, +10 ft move speed (all unnamed), cold resistance 10, superior low light vision, and Darkvision 60ft, you even get the ex. ability to Hide In Plain Sight which is the real winner.
If you need more than 10min/day, you can have a continuous collar for 22,000gp. Just sick.
I'll list more later... your turn.