GUIquotes is a program which will display a quote from a quotes file. It can be set up to display a quote when your computer starts up or you can run the program when you are in the mood to see a quote. This site has more than 60 quote files from an assortment of TV shows.
The program will also work with UNIX fortune format files. There are an assortment of these quotes files on a wide range of topics. The links page has pointers to some sites were these can be found.