Any good hexadecimal tutorials?

Hai.....is there any good sites that teach hexadecimal...i know theres alot already but still very confusing coz all of it don't show good example of applying or using it.

e.g-if i have a program called hex-editor...how do i use  
     it to edit another program and how do apply
     the tutorials from sites for editing the hexadecimal  
     value in a program.