They can also be done using built in effects and masks, as John Baker is suggesting, or you can obtain some of the Pixelan effects plugins as I did. I made a quick video of some of the possibilities. They are almost unlimited. I did mostly flashes but left a few effects on the screen a bit longer. They can all be modified, fine-tuned, animated to move, change, and rotate, and there are many more styles and possibiilties available. You can download and try them out yourself. I will try to post my video here but since I have never done this before it may not work. There is also a link to Dropbox after the image just in case it doesn't work.
There are also hundreds of VirtualDub filter available here, I have been testing about 50 of these and have found only a few which do not work with MEP - these are the ones which try to change the movie size.