Quick Graphviz diagram for GeoServer's codebase

architecture.dot

digraph {
    node [shape=box,fontname=Sans]
    edge [fontname=Sans,fontsize=6]
    Catalog
    subgraph {
        OWSDispatcher
        WebMapService
        WebFeatureService
        WebCoverageService
        WebProcessingService
    }
    subgraph {
        node [fontsize=8]
        edge [style=dotted, label = "Uses"]
        JsonLib [label="json-lib\n(GeoJSON)"]
        JavaxImageIO [label="javax.imageio\n(PNG, JPEG, GIF)"]
        IText [label="iText PDF"]
        Batik [label="Batik SVG"]

        WebMapService -> JavaxImageIO
        WebMapService -> IText
        WebMapService -> Batik
        WebFeatureService -> JsonLib
    }
    subgraph {
        Wicket
    }
    subgraph {
        Restlet
    }
    subgraph { // Flow of control
        edge [style=dotted,label="Delegates\nweb request"]
        Spring
        Spring -> OWSDispatcher
        OWSDispatcher -> WebMapService
        OWSDispatcher -> WebFeatureService
        OWSDispatcher -> WebCoverageService
        OWSDispatcher -> WebProcessingService
        Spring -> Restlet
        Spring -> Wicket
    }
    subgraph {
        node [fontname=Sans,fontsize=8]
        PostGIS
        Shapefile
        GeoTIFF
        MrSID
    }
    subgraph {
        edge [style=dashed,label="Gets\ndata\nfrom",dir=back]
        PostGIS -> GTDataStore
        Shapefile -> GTDataStore
        GeoTIFF -> GTCoverageStore
        MrSID -> GTCoverageStore
        GTDataStore
        GTCoverageStore
        GTDataStore -> Catalog
        GTCoverageStore -> Catalog
        Catalog -> OWSDispatcher
    }
}

I put a rendered image at http://dev.opengeo.org/~dwinslow/architecture.png