When you need to list a handful of companies on a presentation slide, the main visualisation people use is a logo. It always looks great. Make sure you have the latest one (they tend to change rapidly), and pick one in a nice high resolution. If the colours clash too much, consider toning them down by making them black and white.

But the alternative to the logo, is actually getting an image of the company in action. An ad on the street, the neon on the corporate headquarters (no, not the HQ reception desk), a store front, etc. Make sure you don't have any copy right issues. I usually search for photographs on Google Image search that are "labelled for reuse" Below an example for Vodafone:

Image by Moyan Brenn on Flickr: https://www.flickr.com/photos/aigle_dore/15754634911

Image by Moyan Brenn on Flickr: https://www.flickr.com/photos/aigle_dore/15754634911

If you need to deal with a lot of company names, there is no escaping to the logo page. My presentation app SlideMagic makes lining up lots of logos very easy. Use the black and white toggle to mute logos if the colours get too busy.