Floor and Ceiling to specified int
Your methods take int parameters but it seems like you only have really thought about positive numbers. This yields some inconsistencies. For example:


(-28).FloorTo(6) == -28
(-28).FloorTo(-6) == -28
(28).FloorTo(-6) == 24


For CeilingTo the results are:


(-28).CeilingTo(6) == -28
(-28).CeilingTo(-6) == -28
(28).CeilingTo(-6) == 18


If you don't want to think about or deal with negative numbers you should either change the signature to accept only uint parameters (which would require the caller to think about what to do in case he's got an int) or you check and throw an ArgumentOutOfRange exception
Just considering positive numbers FloorTo could be changed to

public static int FloorTo(this int value, int interval)
{
   var remainder = value % interval;
   return value - remainder;
}


Less logic to read and verify in your head.