This is my list.
1st off I consider myself a learner, Gamer, professional etc. I use Mepis as one of my 2 operating systems on my main pc.
These are in no order. Since I don't remember what software came with my default 8.5 cd install, I'll just wing my answers.
1) Gimp - Latest build and keep it updated.
2) Latest Thunderbird as well as Firefox, keep them updated. Thunderbird that is down-loadable from Mepis is not even at version 3 and the community has only 3.01 none the less Thunderbird is not installed by default. I can install other browsers than Firefox Manually.
3) Wine - not sure if that is installed by default, but lets keep up with the latest and greatest build.
4) Audacity sound editor
5) Pidgin chat client. Much better than Kopete. As long as it is the latest build
6) Xchat. Best IRC client out there bar none. Latest build of course.
7) If WxCam is not included, that would be cool. If someone is installing this on a netbook, to have something that manipulates that Netbooks webcam is great. Most Netbooks come with webcams installed. Because this will be on a flash drive and bootable, this would be perfect for a netbook install.

Kget and Ktorrent, Not sure if these were installed by default. The Best download and torrent managers in the business for KDE 4.x.x
9) Pan Newsreader and Klibido. Pan being the best usenet newsreader and Klibido being the best Usenet binary downloader out there.
10) Acetone ISO emulator. Or some equivilant where someone can easily mount and manipulate .iso .bin. que files. So they can run stuff from them.
There may be more but that is all I can think of for now. Again some of these may already be in Mepis 8.5 cd.
Again if more windows apps and games installed and ran in Wine or were Linux native, Bye Bye Windows, but oh well. Dual boot is fine.