There are many good workbooks available for kindergarten children. However with a regular workbook, a child can do each activity or worksheet only once. A good option is to use printable kindergarten worksheets. These are basically digital versions (like ebooks) of worksheets that can be bought and downloaded to your computer from the internet or from CDs and then printed out. The advantage is that you can print out whichever sheet you wish. If you need to re-inforce a certain concept more you can usually print out that sheet again. Of course you will therefore need to have a computer and a printer.
Quality vs Quantity,Some free worksheets are not good quality - the pictures are fuzzy, backgrounds print grey or speckled - and children tend to notice these things. If you are using the worksheets to educate your child, you may want to choose good quality worksheets that encourage your child to produce good quality work. After all, its a little difficult to ask your child to color within the lines and work neatly when the worksheet they are filling in hasnt done the same.