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Using object detection to sniff out different browsers

Since different browsers support different objects, you can use Object Detection as a quick though less than infallible way of detecting various browsers. For example, since only Opera supports the property "window.opera", you can use that to instantly separate an Opera browser from the herd. Here are a few sample test cases, and what they *imply* about the browser running it:

Example objects used to "rough" detect a particular browser
Scheme Detects
document.getElementById Firefox1+, IE5+, Opera7+, Safari, and most modern browsers in general.
window.getComputedStyle Firefox1+ and Opera 8+, and Safari 2+
window.globalStorage Firefox2+
window.globalStorage && window.postMessage Firefox3+
document.getElementsByClassName Firefox3+ and Opera 9.5+, and Safari 3+
document.querySelector Firefox3.5+, IE8+ (in standards compliant mode only), Opera9.5+, and Safari 3+
document.all IE4+
window.attachEvent IE5+
window.createPopup IE5.5+
document.compatMode && document.all IE6+
window.XMLHttpRequest IE7+, Firefox1+, and Opera8+
window.XMLHttpRequest && document.all

or:

document.documentElement && typeof document.documentElement.style.maxHeight!="undefined"

 IE7+

Note: First scheme will fail if visitor explicitly disables native xmlHTTPRequest support (under Toolbar-> Internet Options-> Advanced). The second one will not.
XDomainRequest IE8+
document.documentMode IE8+ (detects IE8 in a specific document mode, such as standards complaint).

Because IE8 can render a page in various different modes depending on the page's doctype plus the presence of certain HTML elements, documentMode is an IE8+ property that returns a different number depending on the mode the page is being rendered in. They are:

5 Page is running in IE5 mode (aka "quirks mode").
7 Page is running in IE7 standards mode.
8 Page is running in IE8 standards mode.
9 Page is running in IE9 standards mode.

This means that even though the user is using IE8 or IE9, for example, if a webpage is missing a valid doctype, document.documentMode will return 5.
XDomainRequest && window.msPerformance IE9+

window.msPerformance is a IE9+ property you can use to detect IE9. There are other IE9 only window properties you can use in place of it as well for the purpose. See a list here.

window.opera Opera (any version)

* Since Opera by default also identifies itself as IE (apart from Opera), with support for many of IE's proprietary objects, the IE detection schemes above will also return true for Opera. Use "window.opera" in combination to filter out Opera browsers.

It's important to mention that object detection's chief purpose is to help you detect within your script whether the browser supports a particular object/method/property before using it, not browser detection. As the later it may be convenient over probing the Navigator object, but is only as reliable as your understanding of which objects are supported in which browsers. In other words, it's not a 100% reliable way of sniffing out the user's browser. Having said that, the below uses object detection to test for IE7:

if (document.documentElement && typeof document.documentElement.style.maxHeight!="undefined")
alert("You're using IE7")

The following detects a page running in IE8 standards compliant mode:

if (document.documentMode==8)
alert("This page is running in IE8 standards mode!")

Again object detection is really about feature detection. It detects whether your browser supports the feature your script intends to use and manipulate. For the lazy, that will substitute for browser detection just fine!