Just like with desktop operating systems, Android handles opening certain types of items with a “default app”. When opening a web page, for example, your default web browser is used to open the link. If you install another web browser you’re asked which app you want to use to open links of that type, and can optionally set one of them as the default app.
To do so in Froyo (Android 2.2), open Settings, Applications, Manage applications, and tap the “All” tab. Next, find the app that you want to change, in our web browser example select and tap “Browser”. Find the “Launch by default” section and tap the “Clear defaults” button. (Don’t worry if yours is grayed out or says “no defaults set”, you’ll only see these when you have more than one app capable of handing the same type of action.)