Here are five timeless favourites: Univers Condensed (Linotype), Avenir Next Condensed (Linotype), Frutiger Next Condensed (Linotype), Akzidenz Grotesk Next Condensed (Berthold) and FF Din Condensed (FontFont). More recent strong contenders include Founders Grotesk Condensed (Klim), Benton Sans Condensed (Font Bureau), Velino Sans Condensed (DSType), Tablet Gothic Condensed (Type Together) and Whitney Condensed (H&FJ).
I have to give a special mention to Gotham Narrow (H&FJ): very space-efficient, but not too obviously condensed-looking and therefore ideally suited to longer portions of text in tight spaces.
Condensed serif fonts are often tiresome to read. A very elegant condensed serif font is Chronicle Display Condensed (H&FJ), intended for titles and headings only.