Windows only: Free application STDU Viewer is an all-in-one tabbed reader for electronic documents, supporting everything from your standard PDF to the less common comic book archive (CBZ/CBR). It's free, fairly lightweight and snappy.
(Click the image above for a closer look.)
STDU has a nice browser-style tabbed interface for viewing multiple documents in one window, supports most electronic document format you would want (including TXT, TIFF, PDF, DjVu, Comic Book Archive and XPS), displays thumbnails of pages in the sidebar and allows for bookmarking in documents. As FreewareGenius points out, it also has a really nice text search tool that displays all the results in a list rather than requiring you to jump all over the document.
It may not have been one of the favourites on our readers' list of the five best PDF readers, but it supports a lot more than just PDFs, and it's a formidable viewer if you're just looking for a change in your default reader.
STDU Viewer is freeware for non-commercial use, Windows only.