Chrome/Firefox/Opera: If you aren't completely overjoyed with Steam already, previously mentioned Enhanced Steam is a browser extension that adds a lot of extra info to Steam in your browser.
With the extension enabled, you'll see additional boxes with info such as how long a game will take to complete, which DLC you already own, and price history information (though we'd still recommend IsThereAnyDeal.com for even better price tracking). It will also automatically bypass the annoying age check and it shows a bright red warning when a game uses external DRM (such as Games for Windows Live). The one unfortunate downside is that it only works in your regular web browser. If you prefer to peruse Steam with the native client and its built-in web browser, you're on your own.
Enhanced Steam [Firefox Add-Ons]
Enhanced Steam [Opera Add-Ons]