Like the guy below you said, Apple's OSX depends on USB devices that use the industry-wide HID standards.
The problem is that some manufacturers will release joysticks that are non-HID compliant because, for some reason, they want to make THEIR joystick super-special, or give it some special proprietary feature. So that joystick ends up breaking compatibility with OSX.
This is not the first thread to discuss this, but from my own personal research, here is a small list of USB joysticks that are HID-compliant, and therefore do work with MacOSX right out of the box. You don't even need to install the drivers that come in the box, and they probably don't even bother to include a Mac driver in the first place. I will put minor commentary for some entries:
Logitech Extreme 3D Pro Joystick (right handed)
It's a straightforward 'cheap-entry' USB joystick. No frills, lots of buttons, feels like cheap plastic, but it's easily Mac compatible out of the box.
Thrustmaster T-Flight Stick X Flight Stick (slightly right handed)
After some research, I found that this stick was actually OSX compatible as it is HID complaint, even tho it says PC only on the box. I bought one, and yes it works. This stick feels more sturdy than the Logitech, IMHO. There is also a HOTAS version that is also proven Mac-compatible, and THAT version happens to be a crowd favorite among the Mac veterans who have been playing X-Plane for over a decade, many forum member were pushing me to buy that one instead. I decided to get the non-HOTAS version because I wanted a simple joystick, not something that requires 4 arms to operate.
CH Products Flightstick Pro USB 4-Button Joystick 8-Way Hatswitch (ambidextrous)
CH has always been a Mac-friendly company. And from the various X-Plane forums, CH sticks seems to be a Mac user favorite. This model is a simplistic few-buttons version, but also one of the very few true ambidextrous flight sticks available. I only have one negative for this stick, that it lacks enough buttons, and does not have buttons to operate the rudder controls (used in the realistic flight sims like X-Plane). You do have the option of buying a "throttle" stick for your second hand or a pedal thinggy (rudder), but the cost does add up.
I won't mention the other sticks like Saitek or MadCat or Microsoft since I have no experience with them, or did not even do enough research on them when I considered a purchase.