The DSi's common key is a known encryption key (using the AES-CBC-128 algorithm). It is used to decrypt the title key in a ticket.
Loopy had substantially claimed to have found the common key, somewhere around 2009, but he didn't share it. He did, however, provide a small amount of information on it. (There are no 00 bytes and the first bit is a 1.)
The common key can together with NUSD be used to decrypt system software only, not DSiWare, because DSiWare tickets like WiiWare tickets aren't available on NUS.(The only DSiWare tickets available on NUS are the free DSiWare, opera and flipnote.)
The common key is *not* in the RAM while either of the currently exploited games are running. The common key is not the same as (and not to be confused with) the Wii's.
Apparently, someone registered the common key as a .com domain the 16 of september 2009 with the following text:
"GTFO NAO, U CANNOT HAZ SITEZ.
DSI COMMON KEY HAS WAN USE. SIGNIN HOMEBREW 2 RUN ON DSI IZ NOT WAN OV THEM. NINTENDO DOESNT EPIC FAIL LIEK SONY IN DIS REGARD.
BTW, OH HAI THAR NINTENDO"
The Common key can be extracted from this program (for legal reasons): CENSORED. This program is not legal because it contains a verbatim copy of the key.