Personally I would recommend and use:
The CookiePal so users can easily delete cookies if they are having trouble at your site.
herePostlist Attachment Icon since so many users don't see the "Attachment" link.
hereMy users didn't want User Ratings.... so I use the Profile Status Bars instead.
hereHope that get's you started.
Oh and one more thing you shouldn't miss... the Integrated Index Page by JustDave. Look inside the Forum design Integration forum. You can see the demo at the Home page here.
:)
I love the "Who's Online Box" additon....although I have modified it so that it appears on every page.
hereProfile email protection from Spam bots keeps the email addresses from being harvested.
hereIf you have alot of users... the userlist is worthless without the Userlist Alphabet Sort hack.
hereThe Inline Images hack is pretty valuable too. It displays attached images in the posts.
here