I've had a look, not as far as I can see. The codes are in different formats and there is no Serial Key in the Steam version that I can find.
Sorry.

Are these any good for practicing design skills while learning "proper" engines and programming etc?

There's a lot of material here. Some are great for prototyping level design (like GameGuru). Some are good at giving you multiple layers and each layer you go, you get to learn more about game-making. But you can get to whichever layer you want to at your own pace (like GameGuru and RPG Maker). Some are programming-only (like AppGameKit) but the programming there is non-object-oriented BASIC. So that will help you learn about variables, functions, loops and whatnot, but not get very complicated as far as variable scope, object-oriented classes and functions or more specific data types.

All, in my opinion, are great for hobbyist or for someone just learning. Because they make the learning fun and they allow you to finish a project.

But if you're going to plan on getting serious into this stuff (like C++, DX library calls, engine building, etc), then you will need to progress beyond these. But these are a great launching point.

Which type of design? Level design or Game design? Anyway, both begins with good old pen & paper as it saves time and you can be more creative when off the computer.

For level design, Stencyl has some tiling and collision system, so you can practice some basic 2D platformer like: how would you do design a Super Meat Boy level?

For game design, RPG Maker is good as you have to think at a larger scale, balancing the stats of your charcheters and monsters (countering debuffs, leveling curve and so on....), the flow of the story and events, the geography of the world, etc...

That's pretty much exactly what I meant, thank you. I was referring to the more "subtle" parts of the game that can't be really determined fully on paper, such as stats balancing, difficulty, speed of gameplay etc that you've to get a "feel" for yourself.
I was mainly going to use these for prototyping and design (as mentioned above), but the detail for each of the programs should be useful to determine which to use and get, thanks.

I bought the full tier, even if I suck at it. XD Eyeing getting the XP version with that coupon. Can you gift the other coupons? (I'm thinking probably not.)

Even if you can they prolly won't let you use more than one coupon at a time.

No. They're tied to your Humble account.

The 2nd week BTA, er, things have been unlocked. They are:

PlayCanvas + Asset Pack + Tankz
Pro Motion 6.5
Humble Bundle Modern DLC Pack
Whisper Of A Rose
Humble Store 50% off Adventurezator coupon

1st post has been updated.

I'm happy to see the modern DLC pack. Looks like it has all kinds of goodies for Guru and RPG Maker and more. And ProMotion looks awesome!

This whole PlayCanvas thing has me confused. I checked out their website and it looks like it's all free open-source stuff with a subscription model somehow attached to it. If anyone understands what you're getting, let me know.

It's a 3 month license.

What with the inclusion of ProMotion, I just ended up buying it. But now I'm confused.
I already own RPG Maker Ace on steam from a previous bundle: http://store.steampowered.com/app/220700/
That shows up as owned for me, and I also have Adventurer's Journey, Royal Tiles and Tyler Warren First 50 Battler Pack DLC for it.

The current bundle says that they have the RPG Maker Ace Deluxe Edition. How is the Deluxe Edition different? I can't find anything like that on Steam. It says it includes the Luna Engine, but that appears as a separate key on Humble from "RPG Maker Ace Deluxe Edition".
Do I want to redeem the key for the Deluxe Edition because it'll have extra stuff, or is it the same as what I already own?

I just tried redeeming the Steam code for RPG Maker: VX Ace Deluxe, and I go the "you already own this product on Steam" response. I bought the last HB bundle they had with VX Ace in it and had already activated it then. I guess it is Deluxe as it also includes the Luna Engine with it too, as I do not already have that myself.

Thanks for the info!
Also, more as a response to my own earlier question than anything else, it seems that the AGF Pro V3 Premium DLC either is not included in this bundle, or is in the $12 tier, because I don't have it, even though I got BTA.

It has been pointed that it is in the bundle. Therefore is in top tier.