I tried to explain the relevant details of your interest below. I want to say briefly, if you want to build great application what you're looking for is that OpenLayers.
OpenLayers is an open source JavaScript library which provides lots of rich method for building
web-based application similar to Google Maps. it supports lots of events and controls beside this you can add too many layers which work integrated with Google Maps, Yahoo Maps, Virtual Earth, TMS, WMS, CloudMade, and XYZ map types, and the MapBox maps.
as MapBox it is really colorful and beatiful map designer beside this helping you to develop microsites with some tools. it's some hard to develop a big application with its tools in addition it is not in bulk. it is really good at map publishing platform however i think limited with capabilities not similar to openlayers. for serving a web map with mapbox, you should check out its tools:
Modest Maps is written to be minimal and make few assumptions about uses. It’s the perfect basis for building mapping tools.
Wax adds zoom controls, permalinks, and much more basic functionality and UI to Modest Maps.
mmg adds flexible support for GeoJSON features in Modest Maps.
HTMAPL makes it easy to make maps with Modest Maps by writing nothing more than simple HTML.
Easey adds easing, zooming, panning transitions to Modest Maps.
as TileMill, it is a map style developer for colorful maps. actually it is a map designer which support lots of file as ESRI Shapefile, KML, GeoJSON, GeoTIFF, PostGIS, and SQLite. you can show your tilemill work in openlayers or google.
TileMill is a tool for cartographers to quickly and easily design maps
for the web using custom data. It is built on the powerful open-source
map rendering library Mapnik - the same software OpenStreetMap and
MapQuest use to make some of their maps. TileMill is not intended to
be a general-purpose cartography tool, but rather focuses on
streamlining and simplifying a narrow set of use cases.
i hope it helps you...
