JSpacers are small oblong rubber o-rings used to convert clicky switches into tactile switches with a unique feel. They were sold on TheVanKeyboards.com.
Traditional clicky switches make noise by utilizing a two-part switch stem with a slider that contacts the metal switch leaf. When the switch is actuated the slider moves past the leaf and snaps to the bottom of the housing, making an audible clicking sound. JSpacers hold the slider portion of a clicky switch in its bottommost position, preventing the slider from moving and eliminating the click. This results in the previously clicky switch operating as a tactile-only switch. The lowered position of the slider removes most of the pre-travel of the stem and the presence of the rubber ring causes compression (also called "preload") on the spring. The result of these changes is a "snappier" and more tactile-feeling press than is available with Cherry MX Brown or Clear switches.
This modification was initially done on Cherry MX Blue switches with standard o-rings and the switches were nicknamed "Jailhouse Blues". The "jailhouse" nickname is what led to the "J" in JSpacers. As a purpose-built product, JSpacers are easier to install and provide a much more consistent feel compared to modifications with traditional keycap o-rings.