Level 0

Once AATSR instrument source data were received on the ground, they were converted to a Level 0 product. Since AATSR had only one nominal operating mode, there was only one type of L0 product, comprising header information and a main dataset containing the source packet information. Full information on the L0 products is available in the AATSR Handbook.
A comprehensive report compiled after the Third Reprocessing performed in 2013, showed the L0 availability over the whole mission to be 97.2%. The report is available here.

A Fourth Reprocessing for the full AATSR archive based on the consolidated L0 data was completed in 2022.

Note, however, that AATSR L0 products are not made available to users.

Level 1A
For the Fourth Reprocessing, Level 1A products now exist as a distinct dataset, before geolocation and L1B formatting.
Note, however, that AATSR L1A products are not made available to users.

Level 1B
More information on the Level 1B [ENV_AT_1_RBT] products  from the Fourth Reprocessing can be found here
The Third Reprocessing AATSR L1B dataset in Envisat format [ATS_TOA_1P], The Gridded Brightness Temperature/Reflectance (GBTR) product contains top of atmosphere (TOA) brightness temperature (BT) values for the infra-red channels and reflectance values for the visible channels  is currently still available from the CEDA archive and can be accessed by all registered users. For new users, access is provided upon registration. 
Data in the Third Reprocessing  archive may have been segregated if it is known or suspected to be of non-nominal quality. Further information on segregated products is available in readme files at the access points to the archive, and is provided here for reference: AATSR segregated data readme.

Level 2 and Level 3
Historic (Third Reprocessing) and external Level 2 and Level 3 datasets are still available from NEODC via the CEDA archive:
AATSR Sea Surface Temperature Meteo Product (ats_met_2p)
The data was originally designed as a fast delivery Meteo product for use by meteorological offices and contains averaged BT and SST at 10 arc minute resolution.
AATSR Gridded Surface Temperature (GST) product (ats_nr__2p)
These data are the Level 2 full spatial resolution (approximately 1 km by 1 km) geophysical product derived from Level 1B product. The contents of the pixel fields, which are a mixture of Top of Atmosphere (TOA) and surface brightness temperature/radiance, are switch-able depending on the surface type.
AATSR Averaged Surface Temperature (AST) product (ATS_AR__2P)
The Averaged Surface Temperature (AST) Product contains averaged geophysical data at two different resolutions, and with respect to two different averaging schemes. Measurement data sets at resolutions of 0.5 by 0.5 degrees and 10 by 10 arcmin with respect to a latitude/longitude grid provide continuity with existing ATSR products. Other data sets contain data averaged over equal area cells of 50 by 50 km and 17 by 17 km aligned with the satellite ground track. Both top-of-atmosphere and surface data sets are provided.
AATSR Level 2 University of Leicester Land Surface Temperature (LST) product (ATS_LST_2P/ UOL_LST_2P)
The data consists of a Level 2 1km resolution AATSR-orbit based product at a pre-defined set of spatial and temporal resolutions. The product provides AATSR-derived land surface temperature data and its associated uncertainty. More information can be found in the UOL LST Product User Guide.
AATSR Level 2 sea surface temperature product (UPA-L2P_GHRSST)
Version 3.0 of this dataset is produced with the L2P processor version 1.2, which replaces the ARC SST processor. The processor applies its own algorithm to generate SST data from the Level 1B Data. This method differs from that used to produce the Gridded Surface Temperature (GST) products. This product also includes the ATSR Saharan Dust index (ASDI) and the clear-sky probability estimated by the ARC cloud detection algorithm. The L2P products are provided in NetCDF-4 format following GHRSST Data Specifications (GDS) v2.
The L2P archive was reprocessed in 2013 with a new processor based upon the ARC SST; the changes are outlined in full in the L2P/L3U Reprocessing User Note and in the SST L2P/L3U Product User Guide.

AATSR Level 3 sea surface temperature product (UPA-L3U_GHRSST)
The L3U product is produced through averaging the L2P data onto a regular grid at 30 arcminute resolution. Hence, the L3U product is similar to the AST/METEO product. The L3U products are provided in NetCDF-4 format following GHRSST Data Specifications (GDS) v2.
The L3U archive was reprocessed in 2013 with a new processor based upon the ARC SST; the changes are outlined in full in the L2P/L3U Reprocessing User Note and in the SST L2P/L3U Product User Guide.

Envisat AATSR ARC Level 3 product (ATS_AVG_3PAARC)
The AATSR Reprocessing for Climate (ARC) dataset consists of Advanced Along-Track Scanning Radiometer (AATSR) multi-mission data which has been reprocessed using various algorithms and in-situ contemporaneous measurements, to provide update retrievals of Sea Surface Temperature (SST) and assess their accuracy. ARC reprocesses AATSR data using new cloud detection and SST retrievals.
The main ARC objective was to reduce regional biases in retrieved sea surface temperature (SST) to less than 0.1 K for all global oceans while creating a very homogeneous record with a stability (lack of drift in the observing system and analysis) of 0.05 K decade.
The ARC project was led by Chris Merchant (University of Reading, formerly of the University of Edinburgh) and funded by NERC and the UK Government.
Further documentation is available via the NEODC website.

All the Level 2 and Level 3 products can be accessed by any user registered for the CEDA archive . For new users, access is provided upon registration. 
The 3rd reprocessing of AATSR data was performed in 2013; the processing updates that have been put in place and the scientific improvements are outlined in full in the User Summary Note for the Third Envisat AATSR Reprocessing.

All the Level 2 and Level 3 product formats are described in the AATSR Products Specifications document.