For Google, you can simply visit Google Trends:
It updates every time something hot breaks loose, you can dig deeper and click on "More Hot Trends" where you can view the most recent 100 top search phrases on Google.com
If a search phrase is volcanic that means masses of people are still searching for it.
For Yahoo, You can simply visit Yahoo Buzz:
This site doesn't exactly show the most popular phrases on yahoo but it does show you what's popular online.
So this is more like a social bookmarking website, similar to something like digg. It updates when ever people bookmark popular stories (Buzz up!), You can sign up and leave comments and connect with other people.
MSN/Live/Bing ...What ever they call it now, I don't think they have a site displaying popular search phrases, the closest you'll get is if you happen to be a adCentre advertiser. You will than be able to use MSN's keyword tool which will show you the number of searches for particular phrases.
But on my opinion, all you need is Google trends. Well since they own most of the search engine market most people who are looking for something will most likely look for it on Google.