What are your favorite Minecraft mods?

Title says it all. What are some of the best mods you've come across in your minecrafting journey? Preferably give a link to its MCForum thread (or wherever its posted) so that others can check it out.

One of my personal favorites is a mod called Metallurgy. Along with adding many new ores, it adds a new method of making ore into ingots. Sadly, it is not updated
http://www.minecraftforum.net/topic/744918-11ssp-smpforge-metallurgy-v13pr2update-21312/

I also really like the Aether mod, but it sadly hasn't updated since 1.0.
http://www.minecraftforum.net/topic/893629-100-aether-v104-01-launch-bug-fixed-bug-fixes-crystal-trees-enchanted-grass-white-apples/

Oh boy, here we go! (Sorry for no links, I'm on my iPod at the moment.)

The Clay Soldiers mod was the first mod I installed, and it was kept installed for quite a while. Epic arena battles abounded.

Mo' Creatures is just great, I love the amount of depth it adds to the whole world. I also love how jellyfish drop slimeballs. I will never need another slimeball in my main survival world...

Flan's Planes Mod is brilliant, it adds freaking planes! And not like those crappy Zeppelin Mod planes. It also adds the ability to bomb things and fire guns in your plane, along with artillery cannons. I never quite got around to shooting down ghasts with that four-barrel cannon...

Finally, the Technic Pack. The Technic Pack is by far the best mod I've ever played. Well, I haven't played all that much of it, as it lagged my computer like heck, but the amount of new content it adds is just phenominal and it just has to be seen to be understood.

inb4 Pony Mod is mentioned

The mod that I had the most fun with is probably the Aether mod, but another one that hasn't been mentioned yet is the Zipline Mod.

http://www.minecraftforum.net/topic/435515-v173-zipline-mod-sspwipvcc/

Because sometimes, simple is best!
Too bad they stopped updating it...

Zombe... Mah fav. http://www.minecraftforum.net/topic/91055-v124-zombes-modpack-26-mods-v61-upd-25mar/

And this one http://www.minecraftforum.net/topic/361430-124smp-smart-moving/ ..Brilliant.

I would've said the aether mod too.

But for this I'll say the Better than Wolves mod.
http://www.minecraftforum.net/topic/253365-124-better-than-wolves-mod-the-mod-that-adds-stuff-to-minecraft-v358-upd-mar-25/

Purely for the functional content it adds!

PokéMobs.

http://www.minecraftforum.net/topic/519098-123-the-official-pokemobs/

It isn't that good yet, but I can tell it's going to be pretty frikking epic.

Smart. Moving. All The Way. :P

The best one of them all, though, is Adventurecraft.

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"Adventurecraft." is a link btw.