Land Products
A selection of Land products are available at the LAADS site described above in the Daily scenes section. For a more complete collection of MODIS Land products you should search for data at either the GloVIS or Earth Explorer USGS sites or the NASA Earthdata Search site. Each site has a full collection of MODIS Land products along with different collections of other data. They each have a unique interface so the method of identifying the data you desire is a bit different. In each case you will select a location, a product, and a date of interest. You need to create a free account at each site to obtain data.
The USGS GloVis site provides an easy, visual display of each tile to help you select your data. First use the image window in the upper left to navigate to your area of interest and then use the Collections tab to select the MODIS product you want. Once you select the data, click the Add button to place it in your “shopping” cart. When finished selecting data, place the order and you will receive emails confirming the order then providing a link to the data. This is a quick process.
Navigate the USGS Earth Explorer site using tabs along the upper left. Under the Search Criteria tab you are offered several methods to locate your site of interest. You can click a point on the map, manually enter coordinates, or upload a KML or shapefile. From this same screen you can filter the data by time. Next select the Data Sets tab and enter MODIS in the Search box or click on the NASA LPDAAC Collections set to locate your specific data. Click on the Results tab to perform the search and view your results. You can add image footprints to the map and view browse images to help refine your search. You can then immediately download the dataset(s) you wish.
The NASA Earthdata Search site has a good tutorial to help you get started. This will help you easily locate the specific data you need for your project.
Atmosphere Products
NASA has a good site describing MODIS atmospheric products, data formats, content, etc. Your first step should be to review this site:
http://modis-atmos.gsfc.nasa.gov/MOD04_L2/index.html
You can obtain MODIS Atmosphere products from the same LAADS site that MODIS Daily scenes are distributed at: http://ladsweb.nascom.nasa.gov/data/search.html. You should follow the same navigation and search procedures as described in that FAQ, only under Group select Atmosphere Level 2 or Level 3 Products.
Ocean Products
Ocean products have been derived from MODIS, SeaWIFS, and other sensors and are available at the NASA OceanColor site. You should read the descriptions of the products and processing levels before working with these data.
Cryosphere Products
The National Snow & Ice Data Center (NSIDC) is the site to learn about MODIS snow and ice products. They have definitive data descriptions and links to data sources. As with other types of data, please read the data descriptions before downloading any data.