Homebrew channel (also known as HBC) and GeckoOS.
HBC is like the windows for wii, most homebrew will need HBC to load (GUI)
GeckoOS will let you play diff region games without brickblocking etc (1:1 copys) and without updating your wii (3.2 is preferred amongst most). You can also load cheats with GeckoOS.
And a downgrader if you want to ... downgrade.
If you downgrade to 3.2 and have kids around. Use the wiikey config disk to turn off local and import updates. Most Nintendo updates try to kill homebrew. As for customization, well their hasnt been much devlopment (everybody was scared of NAND corruption) but with the release of bootmii; i think their should lots of ppl diving in deep soon

. Check out
http://wiibrew.org/ for media players etc.
Dont listen to the muppet above, he just has a raging hardon for the uniiloader atm.