JSpacers are small oblong rubber o-rings that can 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 sliding piece 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 work by holding the slider portion of a clicky switch in its bottommost position. This prevents the slider from moving and eliminates the clicking noise, converting the previously clicky switch into a tactile-only switch. The lowered position of the slider eliminates 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 the old o-ring modifications.