General talk about software patterns

[Jesús Alonso <kenchoweb AT hotmail.com>]

Hi, mate. I'm mostly a game programmer, so my opinion might be biased by my 
field of work, but these are what I find most useful:

Observer: Everytime I need to notify interaction or changes in the systems.
Factory method: More generic factories or even defining the classes in 
external files.
State: Useful for finite state machines. Usually in AI, but I tend to use it 
everytime I have classes with changing behavior.
Facade and Adapter: Wrappers in general terms. Really good for encapsulating 
complex systems in relatively simple classes.
Bridge: Reducing complexity of elements by dividing their responsibilities 
in many different classes.
Mediator: Coordinate the different classes (game elements, GUI 
components...).
Composite: Complex systems made of smaller ones recursively.
Command: I found this specially useful for messaging between objects (in 
game terms: Interacting with other objects)
Singleton: With the help of the Facade pattern, great when you need to make 
many unique subsystems able to interact between them (in game terms: 
Graphics, Sound, Physics subsystems...)

Those are the ten I most use. I've put them as I remembered them, so that's 
the most objective ranking I can provide. Hope that helps.