What is YSFlight

YSFlight is a light-weight flight simulator for Windows and Linux. It was made by CaptainYS. It also has the possibility to play online via LAN. You can also add extra planes, but without there's still enough to choose from. Maybe you should just take a look at the internet site: ysflight.com
Don't forget to choose for the English site (it's also in Japanese). For extra planes etc. you should look at yspilots.com

Hope this helps!