Friends of mine asked me for suggestion for one of their children (age 8) who had bad scores at the local Star test (the family is based in California).
Both parents work, so they have also limited time/energies to go through math exercise with the kid (or may have time only at the end of the day, when the student's energies are depleted, too).
This is not to say that anything requiring parent support should automatically disqualified - it's just to make clear that parent assistance could be a limited resource, so either something that can be done more or less alone by the student, or that gets maximum bang-for-the-buck for the parents time would be preferred.
Books (including exercise workbooks)? Software? Online videos? Games (boardgames, computer games)?
Online videos: Khan Academy has some great resources. Check out their Arithmetic section.
Online game: This is a fun "constructions" game. Maybe a bit advanced, but is pretty coo, nonetheless.
Some general ideas:
Give the kid a jar of pennies, and have him/her make patterns (e.g. rectangles, squares, etc). Start it off with supervision/guidance, and then let them play on their own. Have them count the pennies, first by ones, then by twos, etc. Show how grouping the pennies in stacks of $5$ can make it easier to count without mistakes.
Put up a "hundred chart" in their bedroom (or somewhere they spend a lot of time). Show them patterns like how the multiples of $5$ are in two columns, the multiples of $2$ are in diagonals, the multiples of $9$ are also on diagonals. Show them the Sieve of Eratosthenes to find "prime numbers" when they understand multiplication.
As soon as they know of a multiplication table, show how it can be used to find the number of pennies in a $8\times12$ rectangle.
Introduce them to "recreational math" problems There are plenty here under the recreational-mathematics ranging from super easy to super hard.
Sure: many of these involve some parent (or other adult) help. But that's just because there's no substitute for a parent/significant adult who cares. If the parent takes time out of their busy schedule to (patiently) work with the kid on math, it sends a message: "Math is important to me (the parent), and I care about helping you succeed." Kids (especially young ones) care about what their parents care about.