Take a look at the following folders:
/Library/StartupItems~/Library/StartupItems (if you have one)/Library/LaunchDaemons/Library/LaunchAgents~/Library/LaunchAgents (if you have one)
You should be able to figure out what the StartupItems do by name, but Google them if you're curious.
You can figure out exactly what the Launch Daemons and Launch Agents do by peeking inside their plist files. They'll usually contain a path to something, like a software updater.
As for removing the ones you don't want:
If you see something for a program you've deleted or uninstalled, by all means get rid of it.
If you see something for a program you do still have installed, be careful. These startup items can be as unimportant as an autoupdater or compatibility check, but it's entirely possible that the parent app needs the Item, Daemon, or Agent to run.
So, here's my advice: If you see something from an app(s) (or drivers, etc.) you do have installed, check that app's preferences to see if it lets you disable the function that requires the startup item. If it does, go ahead. If it doesn't, it's there for a reason; don't delete it.
If you really want to play with removing apps' startup stuff, do so carefully and at your own risk. Move the folder or plist to your desktop, restart, and test the app thoroughly. Even if you find it works without its startup item/agent/daemon, keep a copy so you can restore it if something goes wrong.
Testing seems to show that the culprit is Intel's Turbo Boost technology, which disables processor cores on the fly without telling the OS. Since they aren't running the CPU idle process, activity monitor doesn't account for them, and it appears that the machine is under 50% idle when the reality is that it's near 100% idle.
At the moment it doesn't appear as though this can be easily disabled or controlled under OS X: How can I disable CPU throttling and CPU disabling?
Turbo boost is built into many Core i7 and i5 processors, and some of the latest Core i3 processors. You will have to check your processor version against intel's list of Turbo Boost capable processors to find out if yours has it. If you don't have sandy bridge, you probably don't have turbo boost.
Best Answer
From the Apple Developer man pages:
Essentially, this governs what apps, accounts, processes, etc. have access to what services and activities.
I believe wdhelper is a LaunchDaemon that is implemented by, for example, a WD MyBook drive when it is installed to the Mac.