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It's been a while since I've dealt with optical imagery. I need to perform a land cover classification at mid-latitudes using several LANDSAT scenes. I've noticed that some online tools are still selling the imagery, but I was under the impression that the archive was now offered up at no cost.
What is the best/easiest way to obtain temporal Landsat imagery?
Bulk Download
One can also follow the instructions given at Landsat Scenes: Bulk Download, part of USGS' EarthExplorer web-service.
After selecting the scenes of interest within from either the USGS Global Visualisation Viewer or EarthExplorer, one has to save/create the scene IDs of interest as a list (each ID entry should be a single line) in a pure .txt file. More about this USGS' respective Q&A (FAQ): How do I submit a bulk download request for Landsat data?
It only takes then to upload the .txt file in the above mentioned page (web-service) and get as a result down-loadable scenes (links) or submit a Bulk Download order which can be completed by using the Bulk Download Application (see the Help Documentation).
Notes
You can also get it from the Global Land Cover Facility. No registration required.
Check out USGS' EarthExplorer, most are free including all of LANDSAT's archive. You need to register to download data.
USGS is what I use. Note that high quality processing can be achieved in GRASS, I encourage you to carry out atmospheric correction. There are a lot of "primers" out there about calculating NDVI etc which completely overlook atmospheric effects. You can achieve quantitative analysis this way.
I find the USGS NewEarthExplorer to be the easiest way to download Landsat scenes. As I discuss in a blog post, it has a far more 'modern' interface which makes it easier to select appropriate images for your needs.
The USGS have been recommending using EarthExplorer (EE) because it has the most development resources right now.
They offer the USGS Inventory Service as a machine-to-machine SOAP API for communicating with EE for querying their archive for scenes. This way, you can also order scenes for processing to Level 1 georegistered radiance products, and also get the weblink to download them.
A Python tool is being developed here LANDSAT-Download
where you can provide the Python script a list of WRS-2 target sites, and it will download all the available products .tar.gz for the year
For Landsat 8 imagery, I've found the landsat-util tool to be the easiest to use.
AWS hosts Landsat scenes on S3
https://aws.amazon.com/public-data-sets/landsat/
To do land cover classification, you could also consider using Sentinel 2 data. Sentinel 2A has been launched a few months ago, its 10m resolution with 13 spectral bands. The Copernicus program grants full open access to the sentinel data:
Here is an explorer for finding sentinel data
http://satcat.eoproc.com/cat-01/dev-online.html?&config=demo-01
