Formal:
* Clowder: This is the most traditional and widely accepted term.
* Colony: Used for a large group of feral cats.
Informal:
* Cluster
* Litter: Although this is usually for kittens, it's also sometimes used for a group of adult cats.
* Grommet: A less common, but fun, term.
* Herd (used more humorously)
Other:
* Pride: Usually for lions, but could be used for a group of regal cats.
* Squad (for a group of cats you particularly like)
The best name really depends on the context and the group of cats themselves!