Generally, the Joker is left out of solitaire card games as in many others of its type. But there are variations of solitaire games where a Joker does take part, most often as a wild card.
When played in a game of Forty Thieves, the Joker is placed on the foundations while the natural card is unavailable. Any applicable cards are placed over the Joker. When the natural card becomes available, it replaces the Joker, which in turn is placed on the top of the foundation pile. When the Joker is placed on an empty foundation, it stays there until an ace appears.
When played in a game of Freecell, the Joker functions the same way as mentioned above. But when the natural card it replaces becomes available and the Joker is placed on top, the Joker can be placed on another foundation.
When played in a variation of Golf where Kings can be built, the Joker, whenever available, is placed on the wastepile as a wild card and any card can be placed over it.
When played in a game of Klondike, the Joker acts the same way as it is in Forty Thieves. It can also be built while it is still on the tableau. The United States Playing Card Company's version, created by Joli Quentin Kansil, uses two Jokers, with the black joker to be used as a wild black card and the red joker as a wild red card.
When played in a game of Pyramid, the Joker is discarded together with any available card. In this case, the stock is dealt one card at time and can be reused twice.
When played in the game Aces High (or Aces Up) The Jokers are used to clear out a row and are sometimes referred to as "Joker Bombs". When a Joker is dealt into a column, the entire column is reshuffled into the stock and that particular Joker is removed from the game. This leaves an empty foundation slot and greatly increases the win rate.