There was an effort made by the community some time ago to move all the applications and other wonders that were scattered hither and yon across many repositories into just one. That one, central repository is Extras.

Once you enable Extras on your tablet, it should be a rare occasion indeed that you find yourself needing any other repository. That said...

Beyond Extras, there is Extras-devel, which developers use to release beta quality software for testing prior to final release. Software from that repository cannot and should not be trusted to work, so Extras-devel should only be enabled temporarily to install a specific, desired item, if at all.

qole also has his own repository, but that's mainly because he needs to distribute the monstrously huge filesystem images that make his Easy Debian stuff work. It's an acceptable stopgap until some suitable location on maemo.org can be created.

Tear, an alternate web browser for the tablets, also exists in qole's repository, but again, it's an interim solution. There were some technical barriers to placing Tear in Extras, but most to all of those barriers have finally been removed. Therefore, Tear will likely move to Extras, or at least Extras-devel, in time.