I can't beleive no one here has said anything about Elysium.
Elysium is a older game maker that recently has made a big comeback
as an easy, but completely customizable game maker.

It uses Visual Basic 6 coding, along with simple BMP graphics
files and tilesets.

It is a 2D gamemaker, but good 2D.

A standard game would be about 100-200MBs big with it (client)
And the server 20-40MBs.

Although one of the main catches is that you need a good host
to run this. It uses up bandwidth like crazy
(Unless you convert it to IOCP instead of its native packet system)

There are actually several different companies/organizations creating
engines out of it here are the ones I know of:

Elysium (Well known for Diamond): http://splamm.com/elysium/
Eclipse (This version isn't opensource, and doesn't have good rep with other makers): www.touchofdeathproductions.com/
Elysium Blue (Converge with Greentail): http://www.thefmp.com/elysium/

And for those who are wondering, Elysium's definition is: "the abode of the blessed after death in classical mythology"
or: "paradise"

Oh, I used Elysium time ago. It was boring and it was most-likely a dead community which I didn't get, and the games where really odd. O.o;

Despite the fact it didn't work well for me, it was a bit ok, but I'd rather recommend a web search on another kind of program. Elysium bored me to death. :S

Never thought of making a thread about it.