I have a few suggestions for you:
Logistic Pipes - This is an IC2 add-on. It adds expensive pipes that network between themselves. They can be used for intelligent item sorting, auto-crafting, and *very* remote inventory access if you also have Ender Storage or a similar mod.
Applied Energistics - This is one of the last words in efficient item storage and auto-crafting. A well-built AE network isn't as interesting to watch as a Logistic Pipes network, but it can do a lot of the same things in a lot less space. It requires a lot more power to operate however, and can be quite expensive material-wise to get into.
Project: Red - This is the spiritual successor of the now defunct Red Power 2 mod. It adds a lot, but deals primarily with redstone wiring. It adds: a superior redstone wire (which can go up walls and around corners); bundled redstone wires (allowing you to move several different signals down the same block without mixing them); and lots, and lots of redstone logic gates. It also adds several gem blocks and things to make out of them.
Thermal Expansion - In several ways this is almost a replacement for IC2. It can do a lot of the things IC2 can do, but sometimes cheaper or easier. However, in general its blocks can't be upgraded like some IC2 blocks can. The things it is most particularly noted for, however, is the fact that its machines can be configured to pull items from adjacent inventories *without* pipes or additional engines. This makes its ore-doubler easier to get into and run than IC2. And at the high end, it allows you to teleport liquids, items, and or energy across maps and dimensions.
Open Blocks - This isn't so much a technical mod, as it is a collection of small blocks that just make life easier. Among other things is the Filing Cabinet, which stores a large amount of a single thing that doesn't normally stack (example enchanted books). The Jaded Ladder is a trapdoor, which when open acts like a ladder so you can climb to the top of your vertical shaft. The vacuum hopper can suck lose items into it, including XP, making it a choice for mob farms. And speaking of XP, Open blocks lets you drain the XP from yourself into Liquid XP, and then store it in tanks it also provides. It also contains the village trader, which when put near a village will show you every trade available in the village.
Tinker's Construct - This is also not really a technical mod. Instead it is a tool construction/customization mod. Think that your plain diamond pick is boring? Instead smelt yourself up a cobalt pick head, a paper binding, and a netherrack handle and get a pick that is not only faster and more durable than a diamond pick, you can also enchant it with Tinker's materials-for-effects system and repair it when it breaks. It also is just getting into alternate redstone wires and fun things like expanding drawbridges and will soon be getting into custom armor.
