Ok, I'm quite late on that one, TileMill 0.10.0 was released on September 19th. A reminder, TileMill is an open source map design studio by MapBox. We mentioned several times TileMill since early 2011. James goes to the point where TileMill 0.10.0 'changes everything' and Brian use it as an example in his interesting article on GIS vs cartography, saying TileMill brings simplicity and attainable elegance.
From the 0.10.0 announcement: "This release redefines the creative possibilities for web cartography with its new support for compositing layers and features, achieving photoshop-like clipping, masking, blurring, or highlighting. This powerful set of compositing operations can be used seamlessly across vector and raster layers all using pure CartoCSS. The compositing now possible in TileMill, in combination with image patterns or raster hillshades, can enable effects of uniqueness and beauty that go beyond what has previously been possible."
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