Windows only: The Sumatra PDF Viewer is a tiny, open-source, portable, and most of all, lightning-quick replacement for the bloated Adobe Reader we've all learned to replace. It's only a 1.2MB download, so why not give it a try?
Sumatra opened every PDF we threw at it without any issues, along with a table of contents in the left pane if available. You can head into the Options to choose the default layout and zoom, or choose whether to have the sidebar display automatically. Want to copy text to the clipboard? Just hold down the Ctrl key and select the text with your mouse, then use Ctrl+C to copy it. There's even a full set of hotkeys, including Gmail-style navigation. It's not as full-featured as Reader or Foxit, but if all you are doing is reading PDFs, it's definitely worth a look.
With giant hard drives and dirt-cheap memory these days, perhaps the biggest reason to switch to an alternative to Adobe Reader isn't even the bloat anymore — it's the non-stop security holes that seem to plague the popular reader, leaving you vulnerable to drive-by attacks. If Sumatra isn't for you, at least check out one of the other five best PDF readers.
Sumatra is a free download for Windows only. If you plan to keep Adobe Reader installed, be sure to check out the manual for instructions on using Sumatra as your default viewer when reading PDFs from the web.