A tree farm made purely with Botania. Credit: /u/prybulets

Magic, Tech. Naturally.

Botania is a tech mod themed around natural magic. The main concept is to create magical flowers and devices utilizing the power of the earth, in the form of Mana.

Botania is fully playable by itself (and is designed to be played like that), but it functions well with other mods too.

The mod focuses on automation without grinding. By which systems have to be thought out and not just created by following blueprints on NEI or a wiki.

Botania follows some design rules...
No pipes, wires or equivalent, they are boring and overdone;
Abstain from using GUIs in favour of in-world interaction;
Refrain from showing numbers to the player to minimize the incentive to minmax;
Provide pleasing visuals utilizing only two different particle effects;
Utilize or provide renewable resources where possible.

With a few goals in mind...
Break the norm of mods being means to an end, by being engaging in play, not only reward;
Deliver fun and balanced gameplay, without the use of grinding;
Incentivize sandbox gameplay and the creation of fun contraptions;
Provide an immersive experience overall.
Download Botania

Not convinced yet? Check out some videos.

Riskable and Jansey's tutorials are a great way to see what you can do with the mod.
If you prefer a more traditional assessment, try Direwolf20's spotlight. Note: might be outdated.
Looking for a Let's Play? Check out Vanhal's Garden of Glass (optional botania skyblock mode) LP.