A good espresso machine can be a bit of an investment. Before you make your decision, you should know about all the various functions that you can choose from. You may want to get a feature you never knew existed.
Not exactly a high-tech feature. Most 'cup warmers' are just flat space on top of the machine, where it is warm. That's it.
A steam wand for frothing milk is a must-have if you plan on making lattes or cappuccinos. Simpler espresso machines have just a wand, but more elaborate machines will actually have a cannister for milk.
Hot Water Dispenser
A simple but very handy little feature if you use hot water around the kitchen. Saves you from dragging out the kettle when you want tea or hot chocolate.
A little more elaborate than coffee maker controls, good espresso machines allow you to control how much water is used for each pull (and even the amount of grounds, for semi or fully automatic machines that dose automatically). By fine-tuning water and grounds, you can really produce the exact espresso you're looking for.
Different brands will have their own name for this bit of technology. It basically just means that the coffee grounds are moistened prior to your shot being pulled. This little trick will help pull out the aromatic oils for a better espresso.
The lastest thing in espresso making are pods. These little pre-packed discs of coffee grounds make espresso preparation a snap with no mess. Some of the newer models can take pods as well as regular loose coffee grounds.
Many espresso machines have built-in water filters to take out any chlorine or iron tastes from your tap water. A nice addition, but it also adds extra maintenence and cost (replacing the filters).
Integrated Bean Grinder
Not a typical feature, but you can buy espresso machines that have a built-in grinder. These types of machines will usually grind just enough coffee with each use, for the freshest brew around. You'll pay for this advanced feature though.
This feature goes along with the last one, and really only applies to automatic machines that grind as well as brew. If you want to use a different coffee than what is already in the grinder hopper, you can use the bypass doser. A good option for folks who might want to make a cup of decaf once in a while.