There are way too many extensions in the Joomla directory to truly make a Top 10 list since so many of them are excellent.
We do, however, have some favourite we thought we'd share in order to help them get some more exposure.
Here they are in no particular order or ranking:
#1 – JomSocial
JomSocial is an excellent addon reminiscent of Community Builder in that it provides some very cool add on functionality that turns Joomla! into a more social environment. JomSocial has quite a few options and we find it a bit more attractive in that it seems to blend well into the system and provide for a more attractive look.