Globulation 2 is quite innovative open source RTS (real time strategy) game. Innovative means that units don't need babysitting and you can't actually control any particular unit. But let's see how the gameplay actually works.

Like typical RTS, gameplay includes building buildings and units, but the implementation is bit different this time.
You can assign one or more units to a building. If the building is under construction, more builders will gather up resources to finish the building. As you can't directly command any particular unit, nearest free one will come to do the work. If the building is in working order, workers will gather up resources depending on the building type. For example, inn requires wheat, defense tower stones.
You can also repair buildings. But remember that buildings will be out of order while repairs are going on, so it's better leave that defense tower alone while it's under attack and bring some warriors to defend it.
You can also upgrade buildings, for example you can upgrade barracks to train even more stronger warriors. There's also building to make units faster and one for teaching them to swim.
Hive builds units constantly, as long as there's enough food. So there's no need to constantly get more units in the build queue, who needs queue anyway? Ratios in which certain types units are built can be controlled with a slider. So if you need few more explorers, you can increase the slider and higher percentage of new units will be explorers. And if you don't need workers anymore, you can stop producing them. Quite neat, reminds me of the original
Master of Orion.

There are also different kind of areas. One is defence area (blue, warriors gather there, otherwise they roam freely in the base), forbidden area (red, quite handy) and clearing area (yellow, resources are cleared from that area).

But sometimes there is need for more direct action. That's why there are flags which attract units. Most important is attack flag. You can attract only warriors that are certain level or over and assign up to 20 warriors to the flag. It's actually quite neat to sweep whole enemy base with a war flag...
There are also exploring and clearing flag. Clearing flag is more effective than clearing area. Sometimes it's hard to get area cleared if workers are busy, unless you use clearing flag.
Single player game is pretty short, seven mission campaign and number of multiplayer maps to to train with. But at least there's some kind of plot going on in the campaign. Most of the missions are not difficult, but two last ones are more challenging. Last one was actually so challenging that it kept me occupied few days.
Graphics are nice, as you can see in the screenshots. OpenGL is also supported, so game is fast enough with older computers if there's 3D acceleration available. Most important use for the OpenGL accerelation is the cloud shadows, otherwise it's not in heavy use. Intromusic is nice, but music during the game gets repetitive quick.
Game doesn't stop with single player game, as there's also multiplayer available. I have not tried it, but it should be fun.
So final judgement is that Globulation 2 is the best open source RTS that I have seen so far. There's not much single player fun, but at least it's fun while it lasts. Plot is not that special, but it makes campaign feel like one. The game is actually quite fast paced, unless you're out of horse power. If the bar in top right is red and reaches the right corner, it would be good time to switch that OpenGL acceleration on if available.
Globulation 2 is available for the usual platforms, Windows, Linux, FreeBSD and Mac OS X. And as the game is open source, you can also join the development, wiki includes some examples of the possible tasks. One of them should be easy enough, join a multiplayer game