Auctioneer - This keeps track of auctions that it sees and displays information that makes it very easy to figure out what to sell or buy different items for. My biggest problem with this addon is that it gives me lag spikes on occasion. These lag spikes only happen when it loads data, so it shouldn't happen during raids or PvP.
Gatherer - Records where you find nodes for things such as herbalism, mining, and archaeology, and puts symbols on the map so that you can tell where the most rich areas are likely to be. Information is shared to other guildies if they are on when you harvest a node.
RareFinder - Places symbols on your map for every rare monster in Draenor, and even changes the symbol if you have already defeated that monster for the day. Handy for finding specific rare mobs, or for finding rares for Tanaan dailies,
_NPCScan - A handy addon that tells you when there is a rare monster near you, and announces when the 4 special rares in Tanaan Jungle spawn. Can get a little annoying when you are not searching for rares.
Postal - Basically just reduces how much I have to actually type out when sending people mail by saving your most recent mail data. Mildly convenient.