Felix's Hammer Notepad
I had to create new particles for the gel on the ground, but you can see I mostly reused the actual splattering effects from the original gels. One unfortunate side effect of this method is that it's incompatible with normal gels (or rather, it's too compatible: you get both the custom gel and the standard gel affecting the player at once), so I'm working on that as well.
These are the only gels I've created at this point, although I have a few ideas for more. It would be very easy, for example, to make a gel that cancelled gravity on any surface it touched, or even one that went a bit further. Make a trail of this stuff up a wall and you can climb to the ceiling!
Here's yesterday's Hammer Notepad entry: those time-slow switches from Super Mario Galaxy 2.
youtube.com/watch?v=4TbZh28q_b0
The effect itself is pretty simple: lowering the phys_timescale to 0.5 and turning on a color-correction filter (because Portal 2 broke the much simpler mat_yuv command).
The part I'm most proud of is the music swapping. Whenever the tracks swap, they stay exactly in sync (the normal one isn't continuing to play under the slow-time one, it's just a bit quieter). There's a soundscript operator stack for this, to sync the laser-receiver chords, but I don't think it's ever used to interchange two separate sounds in the original game. As such, it requires a bit of finicky Hammer logic to start them in sync. I have to enable the normal one, then enable slow-time, then disable normal, then enable normal, then disable slow-time at the start of the map. That's what adds the odd delay in the sound when the level loads.
When I added that delay I now see that I messed up some of the other commands I used to set up the effect, which is why sv_cheats 1 shows on the screen before hud_saytext_time 0 shuts that off. Sorry about that.
That time-slowing mechanic would make for interesting puzzles I've found that it gets really interesting if you have a player_speedmod set to 2 as well as the phys_timescale set to 0.5, so the player moves at normal speed and everything else is slow.
Did you know that American English isn't even my first language? It's actually [spoiler]British English[/spoiler]
Released maps of note:
[spoiler]Versatile Diversity
L-Circuts[/spoiler]
That's not even necessary, phys_timescale doesn't affect the player. 
I'm uploading the version with the audio at the same time as today's notepad: Gravity Gel! It's not Adhesion Gel, but it lets you climb walls and it's as close as I've gotten.
youtube.com/watch?v=7uOVb7g143Q
These purple gel blobs collapse into swirly energy markers on impact, which emit a gravity-cancelling spherical field!
I would have preferred to keep the blobby particle, but it's VERY expensive to render. The energy thing is much cheaper, so it's what stays on the screen permanently.
Out of the gels I've built, this is the easiest to implement into a custom map. It should (note the "should") even be compatible with other gravity-generating effects, such as a PUNT cube! And as you can see, water washes it off instantly.
Just make sure people believe you that you didn't use noclip... (Issue with one of omnicoder's videos).
I figured the Companion Cube would take care of that, but I'll make another doing what Omni did.
or people could just realize that omni and felix are good at modding and can create these items. o_O
IMO cheats are super easy to spot, but that's just because I've dealt with this engine a long time
New videos! I love having a screen recorder!
The one from today is still uploading, but here are some old ones.
Another recording of the antigrav gel, showing that sv_cheats is 0 and noclip is off. 
youtube.com/watch?v=v5rh8d04hwc
Security Turrets from Bastion, although I wish I had a better model for them.
youtube.com/watch?v=v8Xdnvg6FP4
Various objects from Braid. I showed this before, but the audio was horrible.
youtube.com/watch?v=ikl83twH61k
Today, some progress on the Ice-Nine. When water splashes near it it freezes with a cool effect, but for some reason VALVe removed both methods I know of to change the player's friction from Portal 2. So for now you just go four times as fast.
http://www.youtube.com/watch?v=85lMoVuEJcI
Today I got the Blapperture Multitool working...or at least part of it.
This first phase combines a Gluon Gun, a Tau Cannon, and a Zero-Point Energy Field Manipulator. The lab boys over at Black Mesa built all of these, but apparently never considered duct-taping them all together. Honestly, they call themselves "scientists"...
Due to a glitch in my screencast software that I didn't see until now, this video is in two parts. The first whows the primary-fire (the Gluon Gun), although as you can see it still has a few flaws (it only vaporizes things at the end of the beam, not in the middle).
youtube.com/watch?v=pMWmk7yIUWw
The secondary-fire is a Gravity Gun primary-fire that charges up like a Tau Cannon/Gauss Gun secondary-fire. This lets you control how much force you launch the object with. Just be careful not to overcharge it.
youtube.com/watch?v=mMGXH1LPbPw
This is all built in Hammer, without any scripts. There was going to be a script, but it broke. So now it's pure Hammer. Science from scratch!
The other parts of the multitool are a standard Portal Gun and an Aperture Gluon Gun, which I called a Stasis Gun in my previous video. It's literally a "gluon" gun--you pull the trigger and it puts "glue on" stuff.
Are you using the func_tank method with +use on map startup?
Aperture Science: We do our science asbestos we can!