NEI, Tinker's Construct.
Inventory Tweaks: Try playing vanilla for 30 minutes and you'll be crying for it.
Carpenter's blocks & Chisel: I try not to depend on the too much but in some cases theyre indispensable. If nothing else, the ability to use vertical slabs is amazing. I mean seriously, with all the additions to vanilla, where the hell are the vertical slabs?!? They make adding depth SO much more compact.
Extra Utilities: For one item. The Builder's Wand. Especially when test building in creative mode.
EnderTech: RF item exchanger. Its the tech version of the Equal Trade Focus. Though the focus is a LOT cheaper and available earlier in the game.
EnderIO: I LOVE this mod. Especially in 1.7. Its well designed, properly expensed, and has a low system overhead. My fps crumbles once I start building TE machines. I have not a hiccup in fps with a full EnderIO machine room and multiple farms running.
Mekanism: I love it. I'll be waiting on tweaks since its kinda unusable with the hydrogen changes but thats my go-to mod when I'm feeling techy.