AppleCare - I just
killed my PB. If you can afford it, do it. As an alternative to Apple you can try
Safeware.
CoolPad - I never really found the need for this, but I have friends who are never without them.
iCurve - Again, I've never used something like this, but some find a PB stand like this necessary when using a portable all day long.
PocketMouse Pro - I love this thing. I'm really not into the trackpad at all and if you're doing anything like PhotoShop that's a bit mouse-intensive, you'll definitely need something like this.
Small, retractable cord and very portable.
Saddlebag Pro - I ride my bike to work, so this backpack-style it perfect for me. It doesn't scream "computer case" so I think it makes me blend in a bit more to those looking to steal your stuff. Plus there's this little secret drawer in the bottom that's perfect for holding the power supply, mouse, sunglasses, etc. Very handy.