LRViewer is a standalone viewer for Lightroom image previews that also allows you to extract previews as JPEG files. LRViewer makes no changes to the Lightroom catalog, which means that it can't create any previews that aren't already present.
Both the catalog (ending in "lrcat" or "lrdb") and the associated preview file (ending in "lrdata") must be present. You open just the catalog; LRViewer then opens the preview file automatically. No images have to be exported from Lightroom, since LRViewer accesses the catalog and previews directly.
Select a picture to see its metadata in the panel at lower-left. Double-click a picture (or type `) to zoom it to fill the window, or to the largest preview size available. You can navigate between zoomed pictures with the arrow buttons at the top of the window or with the left and right arrow keys. You get back to the multi-image view with `, g, or Esc.