You can purchase these plugins either on your computer and install them onto the device, or you can buy them right on the device. To do that, you have to connect to a wireless network, then access the archos store through one of many links in the device's firmware.
The device opens the store, using opera...the browser you supposedly have to buy a plugin for. I've never actually seen a device with the browser plugin, so I don't know what kind of functionality you get with it, but if you can live without such things as bookmarks the built in browser seems fully functional. The trick is, Archos only allows you to visit certain pages (the archos store, several video pages specifically designed for the device), and these don't allow you to visit any other pages besides themselves. Also, the URL bar is missing. Essentially, you are stuck with these pages unless you buy the plugin.
To break out of this, I had to find a way to redirect traffic the device makes from archos.com to somewhere else, perhaps my own webserver. I had previously read a page that showed how to prank your neighbors who connect to your wireless network by flipping all their pictures on websites upside-down, or by directing all their traffic to kittenwar.com using iptables. Since my router is running dd-wrt, I can enter iptable commands directly into the firmware to achieve the same results.
I started by researching iptable commands, and found a page right on the dd-wrt website which demonstrated a list of commands. I wrote a single command to redirect archos.com traffic to my own personal webserver,
iptables -t nat -I PREROUTING -p tcp -d archos.com -s 192.168.1.102
-j DNAT --to 18.104.22.168
(note: 192.168.1.102 is the ip my archos gets on my personal network, 22.214.171.124 is/was the ip of my webserver)
Success! Any time I tried to go to the archos store, I ended up at a 404 page on my server.
The next step was to figure out what document the archos is trying to access when it connects. Apparently the starting page is http://archos.com/storewifi/shortcuts/redirect.html?acc=3. I mimicked this on my server, and had a new homepage for my archos.
I now have a "fully" functional Opera browser, which I can actually access other webages with. Time to see what this thing can do.
First, YouTube: Fail. Apparently flash is not enabled in the built in browser, or without the plugin.
Next, Facebook: Partial Pass, It worked, but terribly slow.
How about the Acid 2 and 3 tests?
Acid 2: Full pass! Surprising, but it is Opera after all.
Acid 3: Fail, as the test so kindy told me.
A few other pages showed this browser isn't too bad. One thing that interested me was the "home" button on the right. This button used to take you to the archos content portal, where you could read news or watch videos. However, it stopped working after my iptables redirect.
After checking my webserver's access log, I saw the device is also trying to access a seperate xml file in addition to redirect.html. Apparently the "home" button is setup by the xml document, which allows me direct it anywhere I please.
Among other things I have tried with this browser, I am able to upload any file in the filesystem, assuming I know its full path. Maybe this can be useful for further hacking, who knows. I am interested in hearing from people who do have the browser plugin, and knowing whats different from what I've done. Also if you get this working too, let me know!