From the command line, you need to first find out the process ID of the application you want give more or less CPU time to. top and ps are two command line apps you can use for this; otherwise, you can use the Process Viewer in the Utilities folder.
Then run renice from the terminal with the form
renice priority processID
where priority is a number from 0 to 20; 20 is the lowest priority and will only run when absolutely nothing else wants to run. If you run renice as the root user, you have access to running things in the range -20 to 20; -20 will essentially hog the CPU and let almost nothing else run.