Engineer's Toolbox is a tech mod with a focus on customization. It's based around blocks called sockets which can have modules installed on their sides to serve various functions. Different modules can do everything from simple redstone logic, to generating energy.
You'll find more detailed information on the different modules in the mod's wiki, but this summary should give you a good idea of what's possible with sockets:
There are basic redstone input and output modules for simply passing a redstone signal through a socket on one of it's three basic redstone channels, or controlling a machine. There are also logic gate modules wich perform logical functions using different redstone channels including NOT, OR, AND, NAND, NOR, XOR, and XNOR. Sockets also have 3 internal redstone latches which have their own channels (latch channels can also be used as input for logical functions). There are modules for setting, resetting and toggling the latches. There are also timers for producing repeated redstone pulses, and modules that act like buttons, pressure plates, and even block update sensors.
Items and Fluids
Each socket has 3 internal inventories and 3 internal tanks. There are various modules for inputting, and outputting both from adjacent inventories, tanks, pipes, machines, etc. as well as more advanced modules for extracting from inventories and tanks that don't auto output, as well as modules for directly viewing and interacting with the contents of an internal tank or inventory. These modules are useful not only for passing resources through a socket, but also supplying input for the various processing machine modules.
Engineer's Toolbox uses the redstone flux energy network to power the various machines you can install on sockets, and output power from its various generators. There are modules for inputting and outputting energy as well as modules for increasing the energy storage capacity of a socket from 5000 RF to millions. There are also generator modules which range from basic solar panels, to hydroelectric turbines, to a generator that draws energy from stampeding cows (and other entities). If you install the official addon, GasCraft, you'll get access to even more methods of generating energy with sockets.
Not only will you be able to craft a furnace module, you'll also be able to construct machines that when combined can double and even more than triple your metal output. These include a massive limestone kiln multi-block structure that takes some creativity to automate. You can process the standard ores that you find in many mods as well as a few more unique ones such as Tinkerers' Construct's nether ores.
Engineer's Toolbox includes an automated mining system based on the ancient Roman technique, hushing. Large amounts of water are sprayed at the ground to expose the valuable ores underneath. Of course, the "husher" module you can install on a socket is a little more advanced. When fed with water, or a new fluid, slickwater (lets the husher dig deeper) the husher can mine large areas of land with very little energy input. Slickwater can even be pumped out of the hole when you're done and re-used to dig the next one. There are also block breakers (and placers) which when combined with certain other mods, can be used to create large tunnel bore machines to mine even faster.
For details on how to use the mod including detailed instructions on the modules, and how to use sockets themselves, please visit the wiki. The above is only overview and is far from an exhaustive list of what you can do with the mod.
Below are a few fan-made videos that explain various aspects of the mod in depth: