Let's say you're managing a pay per click campaign for an American client and you live in Canada. You need to get a good idea of the American competition on Google. One problem: you're only seeing ads targeted to your location, Canada. A High Rankings Forum thread discusses the options for circumventing this problem in greater detail.
For Google advertisers, there's also a way of viewing ads from around the world. The Inside AdWords blog says that you can append certain attributes to a URL to see the geotargeted ads.
...you can refine the results page by adding location attributes and values manually to the URL of the ad preview page. Optional attributes include a target country, longitude/latitude coordinates, regions, and cities. In the U.S., you can also set a target ZIP code or designated market area (DMA).
If you're viewing ads on Yahoo! or MSN, you can try a proxy server, according to Ian McAnerin, a Canadian.
Since the serving of geolocated ads are based on IP, you will not see any ads that do not include Canada while surfing from a Canadian IP.
One quick way around this (I'm a Canadian with US clients, too) is to use a proxy that has a US IP. I use Megaproxy, myself. They have a free version that will get the job done for you.
Previous Search Engine Roundtable coverage: Viewing Google AdWords Ads in Other Countries & Languages and Google Offers AdWords Advertisers a Preview Tool for Mobile Google Ads. Forum discussion at High Rankings Forums.