There are many accessories out there that can help beginner players. Let me share one of my favorites, and encourage you guys to share some of your favorites with others!
For the student that is interested in keeping their bow straight, the violin bow-right tool is a metal bar that sticks right across any violin or viola. It works really well and has helped hundreds of my students! The last thing you want to think about when you play the violin/viola is keeping the bow straight. The bow-right takes that thought right out of the picture, and allows you to focus on other important fundamentals. Over time, you will be able to develop proper bow straightness based on muscle-memory with the help of this tool.
Let me know if you guys want some other suggestions or share some of yours with the rest of us!
Until your website, I had never seen a bow right tool. 20 years ago, my teacher taught us to very carefully insert a drinking straw into the top of our F hole, and try to not hit the straw with our bow. Some students learned bow control slightly quicker than others, but it worked! The bow right tool is very cool!