It depends on the airline, the specific flight, the specific aircraft, and the class of service of your ticket.
Airlines use a wide variety of aircraft of different ages for their various routes. Older aircraft typically did not have much in the way of passenger power resources; those aircraft that did often only offered power options to passengers in the business and first class cabin.
As passenger expectations evolved, many airlines have retrofitted older aircraft with enhanced power options. The priority of these retrofits typically went to more important routes (e.g., LAX > JFK) and to the first class and business cabin passengers.
Some of the younger airlines have been more aggressive in offering premium on-board amenities such as power outlets (e.g., Virgin America), individual video screens in their coach class cabin.
If you are flying coach class in some antiquated aircraft from a major carrier on a minor route (e.g., Chicago to Green Bay), you should reduce your expectations for in-seat power.
If you are flying business or first class on a major route, in-seat power would be pretty typical.
Most US carriers have moved away from proprietary airline power receptacles, offering typical household-style two-prong 120V outlets. Don't buy one of those converters unless you can get your employer to write it off. Most planes don't have them anymore, so it would be a waste of space and money to carry around one of those adapters.
Note that even if you identify the typical aircraft use for the particular flight you've booked, there's always a chance that the actual aircraft you board might be an older one that has yet to receive the cabin upgrades. There's also a chance for a last-minute equipment change (meaning aircraft) due to scheduling, maintenance, weather or other aircraft availability factors that puts you aboard a different vehicle.
Hope for the best, plan for the worst when you travel. And what should you expect? Probably something in between, based on typical airline customer satisfaction ratings. If you really want to secure a better passenger experience, buy a business or first class ticket. Coach class is the Great Unwashed.
