There is some other interesting things about 9:
http://en.wikipedia.org/wiki/Casting_out_nines
Also since the topic is about Math tricks, do any of you guy know Vedic Math?
Here are some pages which explain what it is and how it works:
http://www.vedicmathsindia.org/tutorials.htm
http://en.wikipedia.org/wiki/Vedic_mathematics
There also are a bunch of vids on youtube, you just have to search for them.
I hope some of you will find this interesting and or useful.