I second @Connor W's suggestion of using Activity Monitor (or "top" from the terminal) to see what's using the most CPU. Unfortunately, that might not show what's using the most GPU, and the GPU has plenty of potential to burn power.
Here are some additional battery-saving tips:
Make sure you have a Flash blocker installed. I like ClickToFlash. It's Safari-only, but there are similar tools for other browsers as well.
Flash is the most notorious CPU/battery hog, and is often running just for ads. I seem to recall a study that estimated that on a Mac with default out-of-the-box settings, under typical web usage, over 10% of your battery life goes to Flash ads alone.
Most Flash blockers also allow you to load native H.264 video from popular video-sharing websites like YouTube and Vimeo. Mac OS X's native video-playback code is much more efficient than playing via Flash.
Avoid using Flash for long-running services like Internet audio streaming (Pandora, Grooveshark, playing podcasts directly from a web page, etc.). Download a native Mac OS X client, or opt for their "HTML5" version, or opt for a standard format audio stream that a native Mac app such as iTunes can play.
Don't keep your backlight on max if you don't need it. Don't power things via USB if you're not on the power adaptor (don't leave iPhones and iPads tethered, eject your flash drives and 3G cards). Turn off Bluetooth and AirPort when not in use.
All laptops get shorter battery life than advertised. It's called "Advertising for the absolute best case scenario in order to make your product look better". See also: lifetime of appliances, gas mileage of your car, etc – Shinrai – 2011-05-18T16:56:21.027
How much shorter is 'much shorter'? If you're getting 4 or 5 hours, that is probably average, if you're getting less than 2 there's probably an issue. Also, what Macbook do you have? They all have different lengths of advertised battery life. – Connor W – 2011-05-18T17:05:34.847
Reviews of the MBA have indicated that routine usage actually provides better than advertised battery life. ex: http://osxdaily.com/2010/10/30/macbook-air-2010-battery-life/ I'm not saying I'm expecting this, and it's also not relevant to my question; I'm not interested in "is this okay?" I'm interested in "what's the cause of this?"
– Chris R – 2011-05-18T17:06:37.570@Connor Macbook Air, latest model. Projected battery life is ~ 3.5h, instead of 7-11 – Chris R – 2011-05-18T17:07:19.753
In my case it was three Java applets still running after the windows that had spawned them had been closed. 100% CPU on three separate cores (out of four). Fan was cranking hard, and as soon as I killed them CPU usage dropped to <5% and fan shut down. As for why I even have Java installed, long story involved mandatory work webapp that uses it heavily. :( – Adam Wuerl – 2013-05-12T20:10:06.007