Well, since we've started the "Allowed" list already, I'll add my $0.02, and rearrange the list from what I see as the least to most controversial:

• CODE (of course)
• STRONG, EM, B, I
• BR, P
• <!-- (comments - take a look, you might be surprised at what is already there!)
• UL, OL, LI, MENU
• DL, DT, DD (well, I still use them anyway)
• BLOCKQUOTE (I use this often actually)
• TT, ADDRESS
• PRE (perlmonk's code does not always cut it)
• SUB, SUP
• A
• DIV
• HR

Almost all of them should require a matching close code as well.