Imperium
I'm a big fan of the Age of Empires game series (although they're not flash games, and they're definitely not free, so I haven't talked about them here), but it's possible to find similar games at the online flash game sites. On such game is called Imperium. I found it at FreeWebArcade.com.
This is definitely a fun little game. You start on an island, and can choose one of two cities as the center of your empire. You'll need to build resources: food, money, wood, iron, steel, etc, so that you can increase your population, build armies, and conquer the rest of the island. Take over every section of the island, and victory is yours. And how sweet it is, too.
There are levels of complextiy to this game. First, you need to balance your empire's production, imports, and exports to maintain an every increasing level of prosperity. The status bar will show how you stand on various items, whether they are increasing or decreasing. If anything gets into negatives (wood or food, for example), your citizens will start leaving your city, abandoning it and leaving you without workers to produce or soldiers to defend. Defeat will come swiftly.
You'll also need to build a balanced army. Make sure that you establish a mix of sword-soldiers, spear-soldiers, archers, horsemen, etc. An unbalanced army will go down to swift defeat, but a well balanced force will be almost unstoppable.
Overall, I liked this game. The graphics are a bit cartoonish, but in whimsical way that was kind of fun. There is a very detailed help section and tutorial, which will teach you how to play and win, and several difficulty levels to choose from. This game gets a good rating from me, but not from my boss at work....