CartoDB has released two nice new basemaps, which can be used for free in any CartoDB or Leaflet created map. The two new basemaps, based on OpenStreetMap data, are called Positron and Dark Matter.
Positron is a grey-scale map and Dark Matter is a darker, black and grey map. You can explore both the new basemaps on my little Map Tiles for Leaflet map. This CartoDB blog post includes details about how you can add the two basemaps to either a CartoDB or Leaflet map.