The main thing I keep in this binder are my reading assessments (which provide me with my students' reading levels). In the front of the binder, I keep my list of students who are reading with their level from each assessment throughout the year. These are super awesome to look back on and see how much progress a student has made. If I have students for more than one year, I have their data from previous years as well.
I also keep my reading level chart handy, so I have a quick reference of grade level and how to convert my students level to other reading assessments.
Then, each student has a section of the binder where I store my data sheets from their assessments. I have been lazy about copying the actual sheets, so instead, I write down all the accuracy mistakes and responses to comprehension questions on a piece of notebook paper. I store all the assessment data sheets/notebook paper in toploading sheets.
I also store fluency assessments for each student as well.
For spelling, we use Words Their Way. So, I keep a copy each students score sheets from the Bear Assessment in this binder as well as their actual spelling tests.
Another thing I keep in this binder for reference-sake is a list of all the Dolch Sight Words. I have most of these separated into flashcard sets that I use to assess what level sight words each student should be reading.