toy tool for knitting is a Wraps Per Inch Knit-Kard & WPI Tool Kit, from Nancy's Knit Knacks. You can click on the picture to the left to see a really big, but in focus, picture which is too big to fit on my monitor.
With this small, but mighty, tool I can measure the yarn weight (thickness) of any yarn, whether or not the yarn has a wrapper with gauge and weight information. You wind the yarn around the little wooden thing for an inch, and then you count how many times around the yarn was wrapped. You could do the same thing with a ruler; however, the valuable information is on the card...
Based on the Wraps Per Inch (WPI), the card tells you the yarn weight, average gauge per inch, recommended US and Metric needle size range, and recommended crochet hook size range.
For example, I wrapped some sock yarn around the tool and counted wraps. I got 21 WPI. The card told me the yarn weight is Finger ("1", Super Fine); should knit up at about 6.75 - 8 sts per in on needles sized 1 - 3(US). Yep, it's fingering weight yarn. I did knit the socks on #1(US) needles, but I usually get about 9 sts per in.
The back of the card explains how to use the tool and suggests that you knit a gauge swatch, no matter what the card says. Another hint on the back of the card: wrap hairy yarns loosely to allow for bloom; and thick-and-thin yarns should be wrapped for 2-3 inches, count, and divide wraps by 2-3 to get WPI.