Having a good air system in your house is really one of those things in life a lot of homeowners take for granted. You might even assume that most a/c units are standardized in most homes. In reality, when it comes time to install one in a new home or simply to replace an old system there are a wide variety of choices beyond simply central air units. Here's a quick buyers guide to the various alternative types of air conditioning units...
Self-contained and excellent at spot cooling a home. Window units make great supplements to rooms of the house where the A/C unit that cools the rest of your home just doesn't work. This can be particularly true for rooms such as garages that don't have a great deal of insulation. That can also serve as quick fixes as they can be mounted and removed at will should your main A/C system break down at any point.
Portable units are basically the polar opposite of the space heaters you may find yourself using during those chilly winter months. Like window units, portable units are more to supplement your main system rather than replace it. Unlike window units, portable units can be taken anywhere in the home and work great for those stubborn rooms that always seem to stay hot, even with central air present. The only downside to portable units is they tend to be the noisiest option.
Wall units are very similar to window units except for the fact that you have to cut out a hole in your wall to mount them instead. Unlike the others on the list so far, wall units should only be installed by a professional as you have to avoid pipes and other things inside your home. While the installation price can be quite hefty, wall units are amazingly efficient. While the investment might be a little steep at first, you're guaranteed to savings in the long run.
Perhaps the closest to its central air cousin, ductless systems provide a real bang for your buck. They are excellent at cooling large areas without having to essentially remodel your home like you would with a central air system. However, unlike other the other units besides the wall unit, they are not portable. If you're considering going ductless you'll also have to consider getting multiple units to keep your whole home cool.
Hopefully, this short list can help you understand your other options rather than simply going with a central air system. You can consult your local HVAC company (such as CRT Mechanical) to get further details on any of the types mentioned.