The Plugin Site - Free Photoshop-compatible Plugins

Photoshop panels (or more precisely Photoshop extensions) run in Photoshop CS4 and higher.
Some newer ones only work in Photoshop CS5, CS6 and CC. Most panels do not work in Photoshop CC 2014 anymore.
After installing them with the Adobe Extension Manager, you can find them on the Window > Extensions menu in Photoshop.

The Plugin Site

ThePluginSite Panel: Apply all Photoshop plugins created by The Plugin Site in a convenient way.

Russel Brown Panels

Adobe Painting Assistant: Turn a photograph into a painting with some help from this step by step guided panel.
Texture Panel: Add textures to images
Adobe Emailer Panel: Prepping images and attaching them to an email just got a lot easier
Adobe Watermark Panel: Takes all the hassle out of watermarking
Edit Layers in ACR: Convert a layer in Photoshop CS6 into a Smart Object that can be edited with the Camera Raw

Alienskin

Eye Candy Startup: Start each Eye Candy filter with a single click rather than navigating the Filter menu
Blow Up Startup: Gives you one-click access to many of the standard print sizes
Gallery Wrap: Assists you in creating wrapped prints
Snap Art Launch and Preview Gallery: Instant access to the filters as well as previewing a gallery with filter effects
Snap Art Sketch Panel: Designed to work with portraits

GuideGuide

GuideGuide: Create pixel accurate columns, rows, midpoints and baselines based on your document or marquee

CSS3Ps

CSS3Ps: Converts your layers to CSS3

Michael Hale

Mode: Set the blend mode for multiple selected layers
Stack Mode: Speeds up the process of selecting different Stack modes

Lumens

Tych Panel: Automates the process laying out pictures side by side

Dmozon

Timer Panel: Stop watch functionality
Trevor Morris Scripts: Panel that runs 12 scripts
Pen Panel: Save time when working with the pen tool.

barstef

Colorisation: Colorize the comics with Photoshop
SoftSkinning: Make skin soft in photos
Video: A combination of video tools.
3D_Tools: A combination of 3D tools.
Anim_Couleur: Colorize frame-by-image drawing animation in Photoshop
Anim_Dessin: Draw an animation frame-by-frame in Photoshop

Manovi

Expression Media Panels: Read and modify Expression Media and IView specific metadata

Myphotoshopeffects.com

Edge FX Lite: Choose between 8 borders to enhance your photos.
YALE!: Simulates Lomo and Holga cameras.

Sebastien Piconnier

Edge FX: For adding edges, borders and textures. Comes with more than 60 edges presets.

Len White

Painters Wheel: Color picker panel

Jean Louis Persat

Boxes Panel: Draw rectangle and round rectangle shapes
Lorem Ipsum Generator: Put dummy text on our layout

CGIdiot

CG Idiot Panel: For multi-layer actions like find all locked layers and disable lock for that.

ModularGrid

Modular Grid Pattern: Create a modular grid using transparent pattern filling

Rocadero

Transform It!: All the Transform functions in one convenient panel
Selection: No more drilling through menus and sub-menus to get to your favorite and most used selections.

Carson Jones

CI Compositing: Access to common compositing commands, image commands, and batch load functions
CI Preflight: Access to common colour commands, image commands, and batch work functions.

CanLinkIt

CanLinkIt: Link any PSD or image file photoshop files to another

higuchi design

Change Layer Content: Change content layer from fill layer to other fill or adjust layer
PixelAdjustment: The panel to adjust pixel without adjustment layer

The Light's Right

Diffuse Glows Panel: Interface for an action set
Digital Velvia Panel: Interface for an action set
Capture Sharpening Panel: Designed for the first stage of sharpening
Output Sharpening Panel: 43 presets tailored for different output media and resolutions
USM Panel: Extends Photoshop USM command
Sharpening Actions Panels: Interface for an action set
Tone Mask Toolkit Panel: Interface for an action set

Andrew Ingram

Gridmaker

nkurence.com

NKS5 Natural Media Toolkit: Natural media tools in one palette.

Roberto Bigano

False Profile: Brings new features and refinements to the process of assigning false profiles
Floating Adjustments: Restores the Adjustment Layers’ bigger floating windows look and behavior as in CS3.

Dan Margulis