The only thing I consider "essential" is probably a screen saver. I got one from ebay that took me 45 minutes to install, (because of bubbles) and it has been nice other than being hard to install. But my next one will be from Javoedge. I got one for my husband and it installed in like 2 minutes, with no bubbles, and wears very well. Javoedge also has a retractable stereo earphone set with 2.5 mm plug. (most earphones in stores have 3.5 mm plugs).
Something that is essential if you want to use external USB devices is a USB adapter cable. The USB port on the 6000 has a mini A connector, and hardly any devices have a mini A plug. Iriver sells one, streamlinecpus.com and figlabs.com also sell them.
Otherwise "essential" kind of depends on your usage. For instance in the area of networking, I have a wireless router, so I use the built-in wifi, and have no need of bluetooth card (to connect to the internet over a bluetooth-enabled cell phone) or wired ethernet card. However, I do have a 56K CF modem card (Targus PA560) that I got dirt cheap ($10.00 clearance) from Staples a couple years ago, in case I need dial-up somewhere.
If you type a lot, an external keyboard might be essential. I use a Micro Innovations MP-0118 IR keyboard (very small), and also have a Belkin F8U1500 (much larger, but has a dedicated number row) that both work after installing Cwaig's irk-zaurus-j ipk. Check out this link for ipk and pix:
http://www.geocities.jp/bakkeo2004/index.htmlOther 6000 users prefer USB keyboards.
If you need extended battery life, a Pelican psp external battery is inexpensive, (about $20.00) and will make your 6000 be able to use wifi for 5 or 6 hours. (Probably a couple hours longer, but I got tired or being online and turned it off.)