Holsters are one way of carrying firearms, either in the open or concealed. However, there is another category of firearms transport method which is frequently overlooked: Concealing containers.
Generally, containers are not designed for speed or ready weapon availability. However, they do have an advantage in maintaining a low profile and going unnoticed.
In the state of California, locked and concealed containers enjoy a special protection in law. While you still can't take a firearm onto school grounds, into government buildings, etc, the locked container status removes a large number of points of concern from school zones - at least as of the time of writing, in 2009. Provided that the firearms are stored unloaded, and your container is locked, it is generally not a violation of California law to transport an unloaded firearm around in a locked case. Even if that locked case happens to be strapped to your hip, and the key is on your keychain. Also, bear in mind that while California law may allow you to carry your firearm to many places, the vast majority of workplace policies forbid it.
While a gun concealed in this manner will be of minimal use in direct contact situations (i.e. muggings, etc), at no point is a concealed, locked up gun any less useful than a gun sitting at home in your safe.
Specific Container Examples
Attache Cases: Any standard briefcase with a combination lock on it will qualify as a locking container under California law. With practice and some carefully performed internal modifications, one can be readily transformed into an efficient quick draw solution.
Blackhawk Belt Pouch Holster: This is the container I personally use. It should be noted that Uncle Mike's also markets the "Gun-Pak", which appears to be a 100% identical pouch. While my Blackhawk pouch is identical to the Uncle Mike's "Federal" size (their largest pouch), Uncle Mike's markets three smaller sizes as of the time of this writing.
Dillon "Plan B": This is a well documented, widely used concealing firearms container. Unfortunately, I don't have one available for review.