I have several nominations. Panda deserves to be on the list because of its heroic scores on AV testing, often matching up well with the best of the paid AVs.
My little ExeWatch application, which logs all .exes and .bats which come on board my system in a Notepad file accessed with one click, is definitely underrated.
I can't agree so much with brain.exe any more after finding out about malvertising. Now you can get infected not just in porn sewers and crack and torrent sites, but at the most reputable of websites. So until the corporate world fixes malvertising, I think the best possible ad-blocker has to be one of the most valuable and underrated security applications.
In the case of malvertising, ExeWatch can be valuable, because it always makes noise and screen flashes when executables are coming on board.