Gnese is an awesome comic book reader that supports a bunch of formats and screen orientations. I've been using it a lot to reread old tpbs on the go.

My G6 card has a built in media player thingy based off of Moonshell, but I've never used the plain Moonshell, so I can't say for sure how different the two apps are. At least with mine, it can play mp3s easily (no ogg or flac though). I use it mainly for that. Its nice to have a portable mp3 player with speakers built in to carry around the house with you. It also cuts off the screens when you shut the case to prolong battery life and you have limited controls using the shoulder buttons (play/pause, skip track) that you can lock if you're carrying it around in your pocket or something. It also "supports" ebooks, but only in plain text or html formats. Both feature poor formatting and lack any sort of bookmarking or navigation aside from scrolling. I use it to read some newsletters that are about 12 pages long, but it would be a pain in the ass to read a whole book with it.

There is also a AIM client that connects over wi-fi and lets you chat with a touchscreen keyboard, but either the app is buggy or it doesn't like my G6; I've never been able to get it to connect to my router.

Doom was ported and seems to run faily well from what I've seen. I played through the GBA version a couple years ago and haven't had any interest in replaying it, but if you get the itch, the DS version looks good.

Goomba Color runs GBC games great. Almost always full speed with sound so you can replay some of those great GB/GBC games from years ago. I just went through Deja Vu and put Shadowgate on there to play soon. Probably the best emu I've seen on DS.

Don't like the NES emus much. The DS screen is just too low res and a lot of games look really poor when they're scaled down to fit on it. Limited mapper support from what I saw too which makes compatibility an issue.

Honestly, in terms of emus, I think the DS is lacking. Most are still written for the GBA, and the few DS ones around are either ports from existing GBA emus or written from the ground up and still way too early to be much more than a curiosity. I've only tried the NES, SNES, and GB emus though. The GG/Master System ones could be better.