When I was homeschooling, I really liked to supplement what we were doing with pages from the Comprehensive Curriculum series. I can't say I like this for the younger ages, though, since worksheets really tend to be more stifling at those ages (preschool, K), but my son was in 4th grade when we homeschooled, so it was a nice way to practice some of the basic skills he was learning.
I did the cheap thing (as I'm prone to do, being pretty frugal on most days) and bought copies from half.com (this was before Amazon.com started doing their marketplace, so that's a cheap place to get books now as well.)
The versions I found that were the cheapest turned out to be older editions, which wasn't bad, but the clip art and such was definitely dated and looked like some of the stuff that might have turned up on a worksheet back when I was in school. Still, the content of the worksheets was not really affected by this.
I was recently at Sam's and saw that they have the line of Comprehensive Curriculum books for around $8. This beats the current price at Amazon -- $13.57. So, if you have a Sam's membership, be sure to check out the book section. They have a variety of other workbooks available as well. Nothing beats the library for books, of course, but you can't really get the same life out of a workbook from the library, no? Also -- I just checked and the books aren't available through Sam's Club online... so your mileage may vary in finding them in your local store. I'm in the Dallas/Fort Worth area, for what it's worth.
In comparing the old and new versions of these books, I'm finding that they're nearly identical except for the artwork (which is a bit more modern) and something I hated about the old versions has changed, thankfully. The old versions would have math, writing, reading, etc. all mixed together and always there would be a writing page I'd want my son to do, but we weren't ready for the math page that was printed on the other side. It was a pain to try to keep up with all these loose pages that were "half done." Now, each subject has its own section and each section is color coded. Much nicer.
By the way, these books are not at all related to the series by Dr. Kay Albrecht... Comprehensive Infant Curriculum, Toddler, Preschool, etc. That series is a wonderful series, however, for early childhood caregivers or those with infants and toddlers who plan to homeschool and believe that it all starts in infancy. It's very developmentally appropriate and child-centered. (Just FYI for those who are more adult-directed.) I remember back in the day there used to be some kind of giant set of these books and I yearned for them, but I was young and they were over $100. Looks like now the price is much lower and so these would make an affordable and quality addition to any curriculum library for young children.