Below you will find websites, apps, and manipulatives to master pre-algebra. 

Mathigon is a free interactive math manipulatives site for basic, pre-algebra, algebra, geometry...

MathUpgrade from Learning Upgrade - Math App - Grades K - 8  - Pre-Algebra - Algebra - High School Equivalency

Virtual Nerd - Pre-Algebra is a free website with colorful visual explanations by topic.

Math Is Fun - Pre-Algebra is a free interactive illustrated resource organized by topic.

Geogebra is an free website with geometry and algebra tools. 

MathsBot has free online math manipulatives.

Toy Theater has math manipulatives: rods, base-10 blocks, fraction strips...