Do not use Mods from wowui as linked above! unless you have some serious trojan protection, just go read the official wow forums for all the problems people had with this site and hacking thereafter.
Best addons can be find via ACE
www.wowace.com The addons on wowace is updated almost daily and they have a real nice auto-updater program. For adddons outside of the ace collection I prefer to use curse gaming
wow.curse.com also well updated site with links to standalone addons such as Auctioneer and ctmod.
Usualy all these sites have search options to seek specific addons, and they have mods devided into druid, mage, caster, raid etc etc.
I have only played druid up to level 30 ish and hardly needed any mods for that, but some mods I would recomend are:
Decursive (addon to easily dispell curses from raid or party members)
Bartender2 (you always need more buttons and the default bars are just not good eneugh)
Onebag (allow all your small bags to be seen as one big bag, easier to manage)
Quartz (modular casting bar, with lag correction build in, even feral druids may find it usefull to heal now and then

)
Outfitter or Wardrobe (to switch between armour sets and weapons fast, since most druids will have a bag filled with a healing, tanking and dps set)
All other mods I would recomend is for endgame, so no need to go into that.