There are two liquid diluents (or "clay softeners") on the market. One is manufactured by Polyform and is called "Sculpey Clay Softener". The other is manufactured by EberhardFaber and is called "Diluent F".
As the name suggests, clay softener can be used to soften raw clay. As it ages, clay can harden or dry out. This is also an issue if too much plasticizer has been leached from clay. It becomes dry and crumbly. Adding diluent a drop at a time and working it in often rejuvenates previously unworkable clay. Go slowly, though, because adding too much softener can leave you with mushy clay. Use this knowledge to your advantage if you ever need "polymer clay grout" for mosaic work or backfilling carved clay. Simply keep adding diluent until the clay is the desired consistency. And if you ever do over-do it, you can always leach some of the stickiness out of the clay by pressing it between sheets of paper with a heavy book for a weight.
Diluent (pronounced "DILL-you-went") can also be used as glue for polymer clay or to thin liquid clay. Some people even use it to smooth raw clay during sculpting.
Be careful not to get diluent on stiff plastics, such as eyeglass lenses and frames. It can cause some types of plastic to melt.
Look for diluent in the polymer clay aisle of your local arts and crafts supply store or on-line.