Mac only: When you're performing processor-intensive processes, it can really slow the rest of your system down. Free app ProcessRenicer allows you to give specific apps a higher (or lower) CPU priority, so your computer doesn't get bogged down so easily.
ProcessRenicer's main window is like a more detailed version of Activity Monitor, showing you which apps are using the most resources on your computer. Selecting an app and hitting the "Renice Process" button, however, lets you adjust its nice value. A higher value indicates that it's "nicer" to other apps, and thus has a lower priority. A lower value means it is mean and moves itself to the top of the priority list.
Yes, it's a bit backwards, but if you remember why it's called a "nice" value, you shouldn't have too much trouble. It's pretty handy when certain apps (like video encoders or decoders) are stealing too much CPU. Note that this may not be necessary for some apps, which handle this internally; and you probably want to stay away from those non-application processes. Hit the link to check it out, and if you're a Windows user, be sure to brush up on how to do this in Windows too.
ProcessRenicer is a free download, Mac OS X only.