I am sorry for the lack of links in my post but I have posted most of this in the news section or you can find it at gbadev:
http://forum.gbadev.org/index.php , the huge list at DCEmu and dev-fr:
http://www.dev-fr.org/index.php/board,7.0.html (it is French language but should be easy enough to navigate).
The thing I have noticed about DS homebrew is that while general stuff does exist it is often far better to get specialist apps to do the job (there are web browsers but I would sooner use a standalone email client or the google earth port rather than trying to get them up and running)
For file system based stuff (copy, move, paste, rename...) I suggest DSOrganise. On rare occasions it can corrupt the file system so I suggest the linux FAT repair port (I do not think I posted this here but it was at GBAdev not so long ago)
You have an R4 which I seem to recall has autopatching but there is a standalone DLDI patcher as well.
A few apps of late have also gained file swapping and other similar stuff (it is as close to windows as you will get unless someone ports dosbox and gets one of the 3.X line somehow running). Dev-fr and GBAdev are where you will need to go.
As for music and video moonshell is probably the easiest way although DSOrganise and LMG-ng have music capabilities and there was a streaming client or two as well. Video wise there is only birslip's very old video app and Infantile Paralyser's DSM player (quality and filesize are increased quite a bit using it)
Text, there are several (about 15 I can think of off the top of my head). They have various options (book style, searching, varying fonts and I think there was an RTF format as well.....) so I suggest a trawl of a site to drag some up.
Likewise there is Comic book DS which can enable you to convert and read PDF and comics.