So apparently the latest trend in online web games is the genre of "Tower Defense Games". Basically, in these games there's a little winding path wandering through your kingdom, and every once in a while evil baddies will start wandering down the path. You have to place little weapon-laden towers around the path in an attempt to whomp these nogoodniks before they make it to the end. There are a number of different weapons to choose from with a variety of effects. In addition, you can upgrade towers as time goes on.
For some reason, I find these little games engrossing. I'm normally not a big fan of strategy games, but this one is so linear, with just options for weapon choice, that I find it quite accessible.
I started off with Laser Defense, and it's probably still my favourite due to the wide variety of weapons. I like the "techy" feel to it too, what with the graph paper and the lasers and whatnot. Today I discovered Flash Element TD and its beta-test offspring, Flash Element TD 2. The first one is very fun, very nice graphics. Try that first so you'll have an appreciation of the second version. I'm still testing that one, but it has great promise. The creatures have better graphics and more amusing noises, and it looks like the weapons will be even cooler once you get to the upper levels. The second version also has a different mechanic, where instead of the creatures cycling through your pathway, they go to the end and try to steal a "flag" and bring it back. When you finally kill them, they drop the flag where they lie. I think TD2 (at this point anyway) is easier, but loads o' fun.
There's LOADS more of games from this "tower defense" genre; just search your favourite local flash game site, and you're sure to find some fun.