There is no difference. Alestic.com simply lists the same Ubuntu AMIs as ubuntu.com, and those AMIs are published by Canonical, not by me.
I publish the web site http://Alestic.com
I used to publish unofficial Ubuntu AMIs under the name "alestic", primarily in 2007-2009 (Ubuntu ). I worked with Canonical to transfer this work to them and now Canonical has paid employees who publish official Ubuntu AMIs. Canonical builds the AMIs for Ubuntu 10.04 LTS Hardy through the present (Ubuntu 11.10) and into the future.
Since a lot of people came to the Alestic.com web site to find out what the latest Ubuntu AMIs were, I continue listing the ids of the Ubuntu AMIs from Caninical in the table at the top of the web site. I clearly identify which AMIs are published by "Canonical" (almost all of them today) but the myth that I publish all of them keeps reappearing from time to time.
I'm looking at changes to wording and layout on the site to better communicate the truth, while continuing to provide the service of listing the AMIs.
Note: There is one series of active Ubuntu server AMIs that I still publish under the name "alestic". Canonical publishes an instance-store AMI for Ubuntu 8.04 LTS Hardy, but no EBS boot AMI. I copy the image published by Canonical (exactly, no changes) and publish an EBS boot AMI for Ubuntu 8.04 LTS Hardy. There are a few companies that depend on this, but it is not recommended if you are just getting started.
Note2: I occasionally publish public AMIs for specific tasks/configurations. For example, I have been experimenting with public AMIs for running a Git server with gitolite: http://alestic.com/alestic-git/
Note3: I create custom AMIs for various companies, but most of these are private.