I was alerted to another idle game recently and now I'm really into it. It's Clicker Heroes.
Unlike all the others I've played this one runs through flash rather than through HTML so it's not as hackable, modable or data-mineable as most of the others I've played. This also means that it isn't much use to me as a learning tool or a model to base my own code on the way Cookie Clicker was. It also facilitates the game being much more graphical than most idle games, and I think the graphics are quite well done.
I'm not going to bother describing the basic mechanics of the game because it's pretty easy to click the link above, click on things for about a minute, and get the idea.
This is a very well done idle game that finds a good place for the value of idle time vs. the value of active time. How much you can progress by not playing the game is a tricky subject for idle games. You should be able to make some progress, but you don't want it to feel as though time spent actively playing the game is worthless. You want to make people wait - since waiting is part of the game - but you don't want them to wait too much without anything happening.
Clicker Heroes deals with this very nicely by having you progress through areas gated by bosses. While normal monsters die without fighting back, bosses give you only 30 seconds to beat them before they reset. To beat bosses you often use special abilities that have cooldowns. You can set the game to automatically advance through zones but if it loses to a boss it goes back to the previous zone and goes on farm mode until you return. So if you go to work or to sleep you'll be farming up gold and will probably be able to easily clear the next boss and possibly rip through the one after that. But because zones increase in monster health and gold reward exponentially, you don't advance too far in your absence.
Some of the math behind the game seems pretty much perfect and some of it - particularly how the prestige system works - feels a little bit off, but overall I'm very impressed with this game and eager to play more and make more progress. Fortunately right now my Sandcastle Builder game is at a place where I have to idle for very long periods of time, so I have lots of time to click on little monsters.