There are the following:
-Airpro 2000 - quite good in my opinion and it is free. Bad side is that there are not so much features, good side is that there are lots of planes (over 160, from DC-3 to A380), including props, jets and Russian ones, also you can start anytime. You may either lease or order planes, download more landfiles (for free) from website or create your own (this way you could create landfile and fly from your own home city for instance). You can play against up to 3 computers or friends (near one computer). Some realistic things are done here - like jets will be more popular than props, also, there are disasters, which may damage popularity. Game could be downloaded here: http://www.cruiseferry.de/airpro
-Airline Tycoon - a commercial title, but quite cartoonish and childish one, definitely not made for aviation enthusiasts. You are going around airport through offices of tourism agencies, etc. and taking orders. It has quite much features (charters, regular routes, hubs, offices at other cities, buying more gates, getting old planes, changing maintenance funding, buying petrol in advance when it is cheap, changing seats, re-engining, hiring more/less staff, etc.), but it's childish design is quite a drawback (for instance, you buy old planes at aviation museum, you can hire terrorists (from airline called ArabAir) to attack competitors planes (and later this is appening almost everyday), you can for instance lay superglue on ground so your competitor would stick and would be late where he/she is going, etc.). You could probably find this at a discouted price somewhere.
-EfZed Airline 5,0 - also a commercial title and it costs very much, much more than average game. I don't have it myself, but as far as I heard it has very many features. It isn't graphical like Airline Tycoon, but like Airpro it does give pictures of airplanes (however, unlike Airpro here they are edited).
-Aerobiz Supersonic - this title was originally released for Super Famicom gaming computer, thus you'll need emulator (but you'll be able to get this game for free from almost any emulation site). In this title, there are four scenarios set in different times (one of them in future), in each of them you'd need to take the lead in world's market in 20 years. There are 3 other competing companies (you can play against your friends too). The world is divided into 7 regions (Europe, North America, South/Central America, Asia, Middle East, Africa and Oceania), you need to take lead in 4 of them in 20 years. In each region there could be one hub, from which you start flights in that region. This game has quite many options, like buying bussineses in cities, but some of them feels to be unneeded. There are only bigger types of planes and there are not many cities. But otherwise, it is quite entertaining - you also have to bid for slots and compete with prices and expenses, thus you could make a low-cost airline (lowering expenses (wages, maintenance, advertising) to minimum and do the same with prices in most routes). One of biggest unrealisms here is that stored planes doesn't costs to maintain, even in manual it is written that "it is wise to buy more planes than you actually need so you could use them in future".
-Airlines - in this game you play in real time, unlike turns in most other games (except Airline Tycoon), thus you have to conserve time. I don't have it myself, it is quite old and thus could be downloaded from most abandonware sites, like The Underdogs (the-underdogs.com)
-Airbucks - this game has not much planes, but also has some interesting options like for instance setting seats (economy, bussines, first) in airliner (you can see diagram of how your passenger cabin will look). As this is also quite an old game, you should be able to get it from abandonware sites.
-Airlines 2 - quite new commercial release by Interactive Vision. It is made to be quite realistic unlike Airline Tycoon - you play on real world map, buy airliners (although names of airliners are changed (Bowing, Airtaxi, etc.). There are quite many airports, but however game itself is quite randomised and this could be seen as drawbacks for people who likes realism. For instance, airport sizes are different in each game. So, in one game JFK
could be "very large", while in other it might be "small" or "very small", while for example Almaty in Kazakhstan will be "very large". Same changes goes for "poor", "avarage" or "rich" cities (rich cities looks more into quality, while poor - into prices) - one day Chicago might be rich, other time poor. Also, not only that big airliners couldn't fly to smaller airports (which is understandible), but also small airliners couldn't fly to bigger airports for some reason. If you like different game each time, get this, but if you like realism than don't. You should be able to find it and some internet stores for discounted prices.
-Airline - some kind of old German title, probably never released in English. I don't know much about it as I don't have it, however I've seen it for downlading in some abandonware sites.
-AirlineSim - the only available online airline simulation. You can play it for free online without need to download anything. It has probably every plane ever built and each time new one is built, it is added to the game. Also, you can fly to very many cities. However, you are not actually in direct competition with other companies, as each company gets it's own possible number of passengers. However, you can contact other companies, offer them your technical basis at airport, etc. You also can see timetables of each airport, draw symbols for your airline and possibly livery, create website for your airline and get a link to it from your airline's page there, etc. Quite an interesting German game. Look at it at http://www.airlinesim.de
, as I given you info about every airline sim game released, you can choose for yourself
. SOrry if I forgot some game, probably others will correct me.