The vast majority of domestic mainland flights are handled by the country's national carrier Aegean Airlines and its subsidiary, Olympic Air. You'll find offices wherever there are flights, as well as in other major towns. There are also a number of smaller Greek carriers, including Thessaloniki-based airlines Astra Airlines and Sky Express.
There are discounts for return tickets for travel between Monday and Thursday, and bigger discounts for trips that include a Saturday night away. Find full details and timetables on airline websites.
The baggage allowance on domestic flights varies according to the airline and the category of ticket you've purchased and can be scrutinized carefully at check-in. It's usually 20kg if the domestic flight is part of an international journey.