Hi, thanks for the replies. The big user is kernal_task at just over 1 gig, then Firefox at 239.1 (which I am using to write this) followed by com.apple.IconServicesAgent at 106 mb.
I have just rebooted and the memory now reads 11.28 gig and the machine is running smoothly. When I posted the thread, it was very jerky. Memory is now down to 10.9. I can't believe that using nearly 12 gig of ram is normal on an idling machine.
It's totally normal, and designed that way.
If your RAM is free then it's wasted. The OS caches things into RAM to make your machine run faster. For example, your browser cache will be stored in RAM unless you happen to run out, then it'll just be dropped (and loaded if needed).
The very worst thing you can do is use one of those "RAM cleaner" programs that claim to speed up your computer by cleaning RAM so that you have more "free" RAM. All you're doing there is slowing the computer down and wasting the resources that it has available to it.
The only time you should be concerned about RAM being full is if you have a program that has a memory leak (solve by updating the app, or by simply quitting it regularly), or you're working on something that eats up a lot of RAM consistently - then you'd benefit from more RAM.
kernel_task is the OS's kernel (if it were a living being, it would be the consciousness, and the CPU would be the brain). It will always use up a fair amount of RAM, and lots of miscellaneous things are put under its umbrella.
In Mavericks use the activity monitor to look at the memory pane and check the "memory pressure" graph. If this is green, you are totally fine.