There are two main ways of using Choosy's functionality from within your browser and some other tools that can help in specific circumstances:
Browser plugins provide great integration with a browser. Choosy plugins usually add an option to the manu that appears when you right-click on a link allowing you to open the link in another browser using Choosy. Some plugins also allow you to trigger Choosy's browser prompt by holding down a modifier key an clicking on a link in your browser.
The downside of plugins is that they are specific to an individual browser, so you will need to install a plugin for each browser that you regularly use.
There is a list of available plugins on the Choosy website.
Bookmarklets are links that you can add to the bookmarks bar of a browser. Rather than linking a web page they run a small script. The Choosy bookmarklets allow you to open the page you are currently looking at in another browser using Choosy. They don't provide as much integration as bookmarklets, but they will work the same in almost any browser.
You can find the bookmarklets on the Choosy website and drag them into your browser's bookmarks bar from there.
One common use of Choosy is to automatically send links to different Fluid browsers. ChoosyFluid is a tool that automatically detects Fluid browsers on your system and creates advanced behaviour rules to use them.