Similar intensity:
* Seething with anger: This emphasizes the simmering, controlled rage beneath the surface.
* Furious: This is a strong word for intense anger.
* Raging: This suggests a wild, uncontrolled anger.
* Enraged: Similar to furious, but often used for anger caused by a specific event.
* Livid: This suggests extreme anger that shows on the face, often through redness.
Slightly less intense:
* Fit to be tied: This implies frustration and anger, but not quite as intense as "boiling mad."
* Mad as a hornet: This is a colloquial expression for being very angry.
* Ticked off: This is a more casual expression for being annoyed or angry.
The best choice will depend on the specific context and the level of intensity you want to convey.