Freecell is the most popular of the many cell type games.
The four empty cells, coupled with the rule that any cards can be played on an empty Tableau space, make it one of the easier games of Solitaire.
It's great when you want to feel smart;)
Many of the other variants are harder.
Big Free cell, EightCell, Free cell, Forecell, KingCell, Sea Tower, Seven by Four, TwoCell.