HSLayers.Layer.WarpedWMS. You know this: sometimes OGC WMS servers do not support projection of your map, but you still want to display them. Special type of WMS layer, together with server-side python-mapscript based script can do this.
// first you have to define the URL to
// hslayers/source/scripts/warper/index.py
HSLayers.Layer.WarpedWMS.proxy = "/wwwlibs/full/hslayers/trunk/source/scripts/warper";
// now, you can create the layer
var warpedLayer = new HSLayers.Layer.WarpedWMS("Layer name",
"http://foo/bar/wms",
{layers:"water"},
// define the source reference system
// src != map.projection
{srs:"epsg:4326"});
map.addLayer(warpedLayer);
Note
With WMS produced by ArcIMS you usually have problems to reproject it into other projection.