Note that some websites, such as Wikipedia, will refer to various web series (particularly in the Marvel Cinematic Universe and Star Wars franchises) as "miniseries", but I won't be including most of those here, because I think of them as regular series (even if they only got one season and told a complete story). There will be some exceptions to that rule, but the things I include here will mostly be indisputably miniseries.
stuff I've seen
Dora the Explorer and the Destiny Medallion
Dr. Horrible's Sing-Along Blog
Inside Films