My school requires teachers to have Data Binders, but I have been using one for about 15 years. I like all of my students' information to be in one place. It makes it easy for me use during conferences and take to meetings. Some years my binder is thicker than other years, but this year is the largest it has ever been due to the increased number of assessments students are required to take during the school year.
I place my student contact sheets in the front pocket so I can easily contact families. Behind my table of contents (which I seem to constantly edit) I keep all of my passwords because who can ever keep up with all of them?!
Students are working below grade level are required to be placed on Personalized Education Plans (PEPs) so I keep all of their information in one section.
I keep my students' reading levels in a section which includes oral reading fluency, Guided Reading levels, and all of my reference charts from Reading 3D, Teachers College, and Fountas and Pinnell.
In the MAP section I keep my training booklet for when I can't remember what a report is called or what it should be used for. I record progress on charts and also keep signed copies of growth reports from parents/guardians.
I keep track of MAP goals so I know who needs reteach and who can benefit from enrichment.
Everything in my binder is designed to be easily accessible and easily understood. I keep my data current because it ensures I am differentiating for my students' needs.