The Heliopan 49mm #25 Red Glass Filter for Black and White Film is the classic filter for architectural photography. White facades glow brightly; the sky is darkened dramatically and clouds become more impressive. It is also excellent for spectacular landscape photographs with greatly improved distant views.
Try using a red filter combined with a polarizer. On a cloudy day, it will appear as if a storm is threatening. This effect will be even more impressive when used near water.
Heliopan Lichtfilter-Technik of Munich, Germany has changed their black anodized brass filter rings on most Heliopan non-rotating filters to their "slim" mounts. These slim mounts are made of black anodized brass stock and have so little rim in front of the lens (5.4mm) that they will not vignette with lenses as short as 21mm.
Heliopan offers a wide selection of sizes, screw-in, series, bayonet, square and rectangular filters including digital, video, center weighted, graduated, wide angle, Kaesmann polarizers, drop-in polarizers, etc.