Here is a knex balance scale I made a few weeks ago. It's uses a simple pivot point and two arms. It has a pointer at the top along with a dial to tell you how far from balanced it is. It doesn't use many pieces so most people can build it. This is the first one I know of on the site so far. It uses two large baskets to hold the items being balanced. This can also be used for counting items of the same mass. Needless to say this has many uses.
Please tell me what you think.
Killer~SafeCracker

Step 1: Part Count

This doesn't use many pieces, but it's always good to have a part count.

Connectors: #
White 17
Yellow 3
Purple 24
Green 36
Red 2
Light gray 2
Orange 2
Gray clip 4
Tan clip 3
Blue clip 2

Rods:
Red 6
Yellow 2
Blue 2
White 85
Green 58

Odds and ends:
Y connectors 4
Track splice 1

Step 2: The Baskets

In this step you will build the baskets. First off build this circular plate.^{Picture 1}.
Next build these four things.^{Picture 2}They are the same so it should be easy enough.
Now build this ring of yellow flexi rods and green connectors.^{Picture 3}

Okay, Now add the four things to the main plate.^{Picture 4}
Last but not least add the ring made in picture 3.^{Picture 5}

Great, you have built one basket. Now repeat this step so you have two baskets.

Step 3: The Pivot Point

In this step you are going to make the pivot point of the scale. Here are two pictures of the same thing.

Step 4: The Arms.

In this step you will build the arms of the scale. First build this.^{Picture 1} Now build the hinge on which the baskets will hang.^{Picture 2} Then connect the arm with the hinge.^{Picture 3}

Good job. Now repeat this step so you will have two arms and hinges.

Step 5: Hook the Arms to the Pivot Point.

In this step we are going to hook the arms to the pivot point. Just connect the three green/red rods to the half circle connectors.^{Picture 1} Do the same with the other arm^{Picture 2} and there you go.

Step 6: Build the Body.

In this step we will build the main body of the scale. First build these two sides.^{Picture 1} Then add green rods to one of the sides.^{Picture 2} And connect the two sides together ^{Picture 3}

Step 7: Build the Dial

You need to build these two things. ^{Pictures 1-2}You'll need them soon. And slide them together.^{Picture 3}

Step 8: It's Time to Put It All Together. Part 1

It's time to put it all together. First add a yellow/gray rod to the body.^{Picture 1} Then add white rods to both sides.^{Pictures 2-3} Now add the pivot point and arms to the rest of the body.^{Picture 4}

Step 9: It's Time to Put It All Together. Part 2

First add the dial to the body.^{Picture 1} Then add this brown/orange connector^{Picture 2}. Finaly we can add the baskets.^{Pictures 3-4} Do the same on both sides.

Step 10: You're Done!!

Congratulations you're done!!

Now to use it just put things in both sides. You can tell it's balanced when the pointer points to the middle slot in the white connector.

If you have any questions let me know.
Killer~SafeCracker