Listing the current greatest mods for each use out there, which will help you save money, time, effort and also tune your skills up.
Action bars :
I find this selection the greatest action bar mods :
Bartender 3 : This mod is quite good, although it lacks easy binding and binding of keys has to be done through the blizzard menu. 8/10
Bartender3 | World of Warcraft Addons | World of Warcraft @ Curse.com
Bongos 3 : An updated version of the classic bongos mod this action bar mod still is one of the greatest 8/10
Link : Bongos3 | World of Warcraft Addons | World of Warcraft @ Curse.com
Trinity Bars : I personally find this the best of the action bar mods, mainly due to its extremely easy binding. 10/10
Trinity Bars 2.0 | World of Warcraft Addons | World of Warcraft @ Curse.com
Bag Mods :
Bagnon : This is a basic "Make all bags one" with a quite simple although great integrated search for items that are hard to find. 8/10
http://wow.curse.com/downloads/details/2090/
Onebag : For those who want a simple, ready to go out of the box type bag mod... this is for your 7/10
OneBag | World of Warcraft Addons | World of Warcraft @ Curse.com
Arkinventory : Ark inventory has a lot of customization, and requires quite alot of tuning which may not be desired by everyone. Although after tuning it, its basically the best out there. 9/10
ArkInventory | World of Warcraft Addons | World of Warcraft @ Curse.com
Unit Frames :
For unit frames there are only 2 mods that stand out from the rest.
X-Perl Unit frames : Xperl frames have been out for as long as I can remember, It it remains to be one of the best due to its high customistion.
10/10
Pitbull : I personally grant myself a pro of WoW addons, but this mod did confuse me a bit, Not sure if the version i downloaded was corrupt or not.
7/10. Note : I may try it again and re-evaluate it when i have the time.
Will Update more over time.