If you're in the business of creating sites with cutting-edge features, it's frustrating to know that some of your users are going to be packing some ancient browser technology that just isn't up to the job, so why not gently prod them in the right direction?
And here's a really easy way to do it. Outdated Browser is a fantastic plugin, created by Büro, that will automatically check if a user's browser can handle your website. If it can't, Outdated Browser displays a lovely-looking notice suggesting that your user updates their browser. Nothing too heavy-handed; no-one likes to be forced.
Adding Outdated Browser to your site is simple; download the files, include them in your project, paste some HTML into your site and call the plugin, and you're good to go. You can find more details over at Outdated Browser's GitHub page.