SudokuSum

A common Killer Sudoku scenario involves a set of groups that are fully contained in either a row, a column, or a 3x3 square. A 'group' here is a set of cells that are required to add to a particular sum. In these scenarios, a particular digit cannot appear in more than 1 group. As a result, the number of possible solutions for one or more of these groups may be reduced. Fewer possible solutions brings you closer to solving the puzzle. In addition, a 'Filter' feature further reduces the number of possible solutions by allowing you to specify digits that couldn't possibly be in a particular group, or to specify digits that must be in the group.
To set up a scenario, use + and - buttons to create, delete and resize groups, and set a sum for each group with the keypad
Possible solutions are immediately calculated, and displayed. Digits are displayed when they're valid for at least one solution. They are displayed in bold when valid for the selected solution. They are underlined when valid for all solutions. Solutions also include information about cells outside of the groups. For example, if a digit is in some group for all possible solutions, then it can't appear in a cell outside of the groups.
A 'Save and Recall' feature allows you to quickly return to previous scenarios as the puzzle develops and additional filters come to light.