Anger, IMHO is not a sin, since Jesus did get angry.
However, when we choose to hold onto our anger (nursing grudges, refusing to forgive, pouting, etc.) it become bitterness which we are commanded to lay aside.
I think that uncontrolled anger is a sin, as letting your anger get out of control leads to hurtful actions and words that are sin;
Ephesians 4:31 NIV
"Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice."