For my web map application http://ian01.geog.psu.edu/geoserver/www/pa/pa3.html I have three levels of background imagery as the user zooms in. At small scales I'm using NASA Blue Marble and a large scale I have 1ft USGS orthos which are both great. However at medium scales I'm using Landsat imagery and it looks poor (to say the least), so can anyone suggest a good source of free imagery for the US (this case is PA but I need to do other states too). Preferably WMS servers.
One option is to try another WMS of Landsat, such as the 'visible' style provided by NASA. There are a number of pan-fused 15m versions of the data, see the Landsat 7 wikipedia external links. However, I don't think any of these are free for public consumption.
Another option is to generate your own Landsat merged images with the quality you'd like, its easy enough to do: first download the tiles you're interested in (you'll want the raw bands, not just the band combos), then use something like GRASS to perform a band merge, using for example
i.landsat.rgb (doc) or
It looks like you are serving up from GeoServer locally for some of your data. You can download the NAIP imagery noted above, and then downsample it if your disk space requires this. If you have the disk to spare, you can just create some overviews of the data (using gdaladdo). If you just want to use someone elses WMS they will do it for you as you request tiles for those zoom levels.
Although I am not sure this would constitute "medium" resolution, here is the GetCapabilities for NAIP 1m imagery for the continental US.