Tardisode
Tardisodes were mini-episodes which accompanied each episode of Series 2 of the BBC Wales version of Doctor Who. They were each approximately one minute in length. Downloadable via the internet or by mobile phones, they were initially made available in the week prior to the initial broadcast of each accompanying regular episode. As no Tardisode was made for The Christmas Invasion, there were thirteen Tardisodes in all. (That said, the Children in Need Special mini-episode that was broadcast in advance of The Christmas Invasion served the same purpose as a Tardisode, although it is not considered one.)
Scripted by Gareth Roberts, these featured different production personnel than the series proper. However, they were all directed by prolific Torchwood director, Ashley Way.
They did not feature the Doctor or Rose Tyler, but focused on other characters and monsters. Most served to set up elements of the episode's story line; in one case, School Reunion, the Tardisode was a direct prequel, explaining how Mickey Smith recruited the Doctor and Rose Tyler to investigate the school in question.
The BBC chose not to include the Tardisodes with the DVD release of Series 2. This, coupled with the fact they are no longer available online as of 2008, renders these mini-episodes officially lost. Though they have continued to be available on video sharing sites such as YouTube.
Update: Even though the links to the tardisode's have long since disappeared from the official BBC site the tardisode's are still within the BBC media selector. Links to these media selector tardisode clips can be found using this site.