I would suggest the following apps.too many to list but i have included a few of my favorites
Launcher pro + a home launcher the replaces TouchWiz & has a built in set of widgets, as well as cool transition effects, & ability to customize the quick launched apps on bottom of screen (you actually get a total 15 shortcuts on a scroll-able quick launch dock. cool widgets include the calendar widget.
Beautiful widgets gives you the big clock weather widgets that look like what most HTC phones have
Handcent- sms messenger app, blows away stock app with all the customization, you can customize individual sound and visual notifications for each contact you test.
Shazam or Soundhound- lets you identify songs by letting device hear & then "tag"unknown songs you may hear.
music apps
There are dozens of good ones ,Samsungs Epics built in one is ok..
PowerAmp is a popular choice I use Winamp, & Meridian i think both have good User interface and are free, Also I use Google Music to stream my music library from my PC on my Android device.
For Video I suggest mVideoplayer
Podcasts Googles Listen or Doggcatcher are excellent choices. Doggcatcher plays video net-casts
Ringdroid- is a ringtone app , select any mp3 or sound file on your device and crop/edit it to create a custom ringtone/notifications
File explorers.. The built in one is adequate but ES File Explorer or Astro are more robust and you can manage, (copy, paste, move,search,rename) & manage multiple files in the depths of your device in a better way.
Some other fun non essentail apps. too many to list but i have included a few of my favorites
Translate- pretty explanatory what it does.
Convertpad- unit converter
Smart measure-
Ultra Voice changer- record any voice and play it back in a library of disguised voice sounds.