Using a Number Line

The BugbrainED educational number line app provides a nice introduction to multiple ways of using the number line as a tool for addition and subtraction strategies.  The number line tool allows children to develop meaning for the operations and encourages the use of counting on or counting back strategies.  It includes tutorial and practice problems, and is still very low cost.
This app meets the common core state standards for Kindergarten and 1st grade, and will allow students to keep up or catch up to their peers.

CCSS.Math.Content.K.OA.A.1,
CCSS.Math.Content.K.OA.A.2, CCSS.Math.Content.K.OA.A.5
CCSS.Math.Content.1.OA.A.1, CCSS.Math.Content.1.OA.B.3, CCSS.Math.Content.1.OA.C.5