By economic strategy game, do you mean a Real Time Strategy game with a focus on resource management (e.g. Settlers, Anno), City Builder games (SimCity, Children of the Nile) or Tycoon games (Transport Tycoon, Hospital Tycoon)?
If you like the looks of Anno, I'd recommend you check out Unknown Horizons. This is a free open source clone of older Anno titles. It's still in development but is quite playable.
For city builder games, I'd recommend reading a post I made a while ago. It's not pretty, but it is a great starting point to finding something you might like.
For tycoon games, you'll need to give a bit more information about what you are looking for. You might enjoy browsing through [/r/tycoon](/r/tycoon), as it was only recently put together and most posts are basic information about popular tycoon games.
You might also have a bit of luck browsing through [/r/gamingsuggestions](/r/gamingsuggestions), as similar questions have been asked there.
Good luck on your search.
Check out OpenArena if you haven't already.
Also open-ig (java) is not bad if you have played Imperium Galactica before. Unknown Horizons is good too if you like resource management / economy.
If you like rogues, Pixel Dungeon (java).
I would suggest you look into FIFE, an isometric engine. The engine itself is written in C++, but the games are written in Python. Two notable projects use it, Unknown Horizons (kind of like open source Anno 1602) and Zero-Project (kind of like open source Fallout 1-2).
The Anno games should fit that bill, especially 1701 or 1404. I think you can more or less turn combat/war off in the free play options and the amount of stuff you can build that depends on another is absolutely massive.
Apparently, Unknown Horizons is a good (and free) knockoff of the original Anno 1601. Never tried it but since it's free you might want to give it a try.
It's not Anno 2070, but if you liked the Anno series and look for something like that, have a look at Unknown Horizons ( http://www.unknown-horizons.org/ ).
The project is a little slow on the progress made lately, but it's fun and the developers welcome everybody to give some feedback or contribute.
Closest I could think of that isn't mentioned yet is Unknown Horizons. Be warned, though, that although it's quite matured enough already, it's still somewhat rough on the edges. Also, you might want to try Simutrans as well, if you'd like to give transport sims a shot.
This is just my opinion, but to me Roller Coaster Tycoon 3 seemed to lose that certain something that made 1 and 2 such great games. There's a demo available if you want to try it out yourself first.
As was posted below, OpenTTD is a must if you enjoyed Transport Tycoon. Some other games that haven't been suggested yet are Unknown Horizons, which is an in-development open source Anno type game, and StarTopia which combines space station management with a sense of humour similar to that of Douglas Adams' Hitchhikers Guide.
>similar in visual/gameplay style to Fallout
The Fallout engine has an open-source clone in the form of FIFE that can run Fallout, if you want something very, very, very similar to Fallout (the earlier Fallout games, like 1 and 2, which I assume is what you're referring to). This is a 2d engine.
Apparently, there are some people making an RTS based on this engine called <em>Unknown Horizons</em>, so I assume that it's clean enough to use for new games.
Note that FIFE is GPL, which is good if you want to make an open-source game, but may not be what you're looking for if you want to make a closed-source game.