More frequently than one might assume, application processes under Mac OS X stick around longer than they're welcome. Even though you've ostensibly quit an application (it no longer appears in the Dock or is turned off via a preference pane), its core or periphery processes may still be lingering and consuming system resources unnecessarily. The benefit in speed brought about by ending these processes can be dramatic, especially on systems with less RAM.
There are other side effects of unnecessary processes that are hidden from plain view. Processes that are using data stored on removable media and mounted disk images can prevent those media from ejecting.
There are two ways to quit processes that are hidden from the Finder: Use the Terminal to list all (or a set of) running processes and their associated PID (process ID) numbers, then use the kill command to end them or launch Activity Monitor (located in Applications/Utilities) and use a graphical interface to identify and end them. The former is powerful and highly configurable, but not without using a multitude of commands to filter and sort and detail the process list. The latter affords easy access to information about memory/processor usage and the files in use (these can include fonts, caches, intra-application executable binaries and more) by any running process.
Here's what the primary Activity Monitor window looks like:
Types of processes that should be considered for elimination:
Processes with names similar to closed applications While there are a few independent application-related processes that should be left running even when an application itself is closed (iChatAgent, if you'd like to be notified of incoming chat requests while the application is closed, for instance), most processes with names similar to applications you thought had been completely quit can be safely killed via Activity Monitor [see box 2 in screenshot].
PowerPC processes on Intel-based Macs Running Rosetta applications chews up large amounts of memory. Whenever Rosetta is active unnecessarily, it's a tremendous waste of resources. In Activity Monitor, click on the Kind tab. [see box 1 in the screenshot] This will split the list into PowerPC and Intel processes.
For instance, we found a PowerPC ATI-related process, the side effect of attempting to install and run PowerPC-only graphics card control software; and an unused FontAgent Pro process running on an in-house MacBook Pro.
diskimages processes Sometimes, the only way to unmount a stubborn disk image is to kill its associated diskimages process. Use the Process Name tab [see box 3 in the screenshot] to sort as such, and look for these processes. Next, click on their name and press the Inspect button. Click on the Open Files and Ports tab, then scroll to the bottom of the text window. You should see the location and name of the associated disk image (e.g. /Users/bwilson/Desktop/X11Update2006.dmg).
Note that you may need to force diskimages processes to quit. The option force quit is shown after the Quit Process button is clicked.
High CPU-usage processes Click on the CPU tab [see box 4 in the screenshot] until its arrow is facing downward. Look at the top of the list for processes that are using extremely high processor resources (sometimes above 100%). These may be hung or otherwise erratic processes not associated with a running application. If they are associated with a currently running application that is hung or not responding properly, quit them (by force if necessary) then re-launch application.
Duplicate processes Click on the Process Name tab [see box 3 in the screenshot] to organize items alphabetically, then look for items with the exact same name then kill all but one.
Note that it can be dangerous to arbitrarily kill processes listed in Activity Monitor. Some are critical system functions, which can cause loss of unsaved data and other problems if forced to quit. Keep an eye on MacFixIt for coverage of processes owned by the system that can be safely quit to work around troubleshooting issues.