Between Mapnik, Cascadenik, and Carto, you have a few options for rendering OSM data to tiles. Here are several open source stylesheets to get you started.
Toner - Transitions between Natural Earth and OSM data in a high-contrast B+W (black and white) map tiles. They are perfect for data mashups and exploring river meanders and coastal zones. Available in three flavors: normal, no labels, only labels.
OSM Solar - Designed for limited storage applications that require extensive areas of pre-rendered maps.
Terrain - Orient yourself with our terrain tiles featuring hill shading and natural vegetation colors. These tiles showcase advanced labeling and linework generalization of dual-carriageway roads. Background reading
MapBox Streets - tk tk tk
MapBox Light - tk tk tk
Default OSM.org style - Matches OSM.org but best for technical OSM data exploration, not map display/browsing. Scroll down to: Rendering with Mapnik to get the XML checkout out of SVN.
Acetate - simple basemap designed for quantitative thematic visualizations.
High Road - Makes it easier to draw the road bits by hiding complicated PostGIS queries behind views, so that you can declutter your layer names while ensuring that you’re using the right selection of roads for each zoom level.
Transit Map in Tilemill (Carto) - A nice carto stylesheet for rendering transit maps using the MapBox stack. Includes a nice map legend!
Maki - From MapBox, includes 64 different symbols at 3 sizes each. 192 total icons and growing.
Airport icons via Mile High Club - From @nvkelso, includes ~14 icons with 60 flavors.
Ships icons via High Seas - From @nvkelso, about 25 icons.
Matt Amos's Map Icons - 35 map icons. Transport, emergency, services, entertainment, and landmark.
Symbola Font Symbols - This font allows you to manage symbology in the datasource and helps reduce duplication in your carto/mapnik stylesheet. Using a font for the icons also lets you easily apply all of the standard Mapnik text effects like text-fill to change the icon color in TileMill. The is a font made from public domain SVG's including: MapBox Maki, AIGA, and National Park Service icon sets.
IAN Symbol Library - The IAN symbol libraries currently contain 2604 custom made vector symbols designed specifically for enhancing science communication products with diagrammatic representations of complex processes.
For small icons (ideally 10px x 10px to 32px x 32px):
For large icons (100 px or bigger display)