Free radar (SAR) satellite data

• You can get Sentinel-1 data from scihub.esa. Requires only registration (And most likely, non-commercial use). As Sentinel-1 has just become operational the archive is not very extensive but should grow quite quickly.
• You can set request data-access propospal on Alaska Satellite Facility. Some data open access. For ALOS-PALSAR you must be a resident of the United States to receive approval for data access.
• You can get data from UNAVCO SAR data archive.
• And you can get data from earth.esa.int. To access the data, you must provide a project. To preview scenes, use EOLi esa. ENVISAT ASAR the best option.

Only managed to find a couple of sources for SAR images and data:

• You can download SAR images from here which are mostly focused on ecological sites such as forests:

• You can download SAR samples from here which contain fairly large datasets (note: the last 4 links at the bottom of the SAR section are dead)

JAXA have made global L-band SAR mosaics at 25 m spatial resolution available from the PALSAR sensor:

http://www.eorc.jaxa.jp/ALOS/en/palsar_fnf/fnf_index.htm

Registration is required to download the data.