Enigma (for developers)
Great tool for image extraction. It lets you extract multiple layers or selection into a file. You can set up extraction directory and choose to extract PNG or JPG. The coolest part about it is that the trim takes place automatically. The way I use this - I normally used to crop close selection, then trim the selection, then export it, then undo, undo. Doing that 10, 20 times per file might be time consuming. This plugin allows you to pick your selection, click the button and continue.
CSS Hat (for developers)
This is pretty cool CSS / SASS / LESS / SCSS extractor. If your photoshop files were styled with blending, you can simply click on the element and then the CSS Hat button. It will give you the ability to copy the style with a click of a button. It works with regular objects giving you rounded corners, shadows, background colors etc., but it also works with text, giving you font style, font family, weight, size and so on.
Subtle Patterns (for designers)
Such. a. time. saver. I used to try to apply different backgrounds for my designs. I would browse through some tiles, then convert them to objects, and to pattern overlay and just say "Nope, that's not it". Rinse and repeat. Subtle patterns has a plugin that allows you to automatically swap between different backgrounds. Set design background, or a background of any element with a single click. Sweet.
Lorem Picsum (for designers)
For many years I would have simply leave empty boxes where the design was supposed to have an image. Did not look well. After that I decided - hey, let's placehold.it. I still use that for a clean look and indication that screams "hey, I'm a placeholder, once you get some media you can replace me". Well, if you want some actual media and color instead - use Lorem Picsum, which will automatically fill in your placeholders with images of animated films such as "Nemo", "Rio" or "Up". Works great for me.
Blend Me In (for designers)
Offers you searchable collections of icons and gui elements that you can just use in your projects. It's very simple to use, however it is a bit slow. Still, it beats browsing through your other projects looking for that scrollbar or checkbox that you want to reuse right now. As of recently you can save your own collections so a little bit of work is required, but in the end it's really worth it.
PixelDropr (for designers)
Not quite the same as the above, but worth mentioning. It works much faster for me, and the interface is nicer and cleaner (I like things nice and clean) - you have predefined sets with 30 slots that are easy to fill up. Then you just reuse your collection in new designs. Pretty great.
Specking (for designers and developers)
This works well for both designers and developers. If you're a designer, you can use it to ease the workload for the developer or to ensure that they follow your instructions to the pixel. This tool is pretty amazing. Might be a little slow on larger files, but it's totally worth it. You select a layer (or bunch of layers), click the button and voila, it creates a new folder for you with nicely drawn out specifications for sizes, colors, fonts, etc. You can set up the properties of what you want shown and using what colors. Really good stuff.
Preview Fonts (for designers)
This would be my favorite tool were it not so slow. I'd like to think that my computer is a beast with 8 processors and 64GB of memory, so browsing through the fonts shouldn't be so slow. I guess I might have way too many fonts. Anyway, if you don't have thousands of fonts like I do, this is a great little tool that shows you a preview of each font installed on your computer.
Flat Icon (for designers)
I use this one ALL THE TIME - whatever icon you may need for your design, just type it in, and it will search its databases for appropriate matches. Sometimes matches that come in are weird, but other times it's spot on and exactly what I need. What I used to do is go to iconfinder.com or one of the other cool icon websites and do a search for each icon. This plugin returns only flat icons, so that you can style them however you want. Totally awesome!
Pixel2Vector (for designers)
Ever need a larger version of something that you created without vectors? Maybe it's somebody else's files and all the objects were rasterized? Well cool. This plugin comes with an action file and it allows you to convert a layer into a vector. It's not exact, but unless you want to recreate every single object from scratch, this one is a life saver. Got an icon that you need to adjust a little - convert and adjust. Building larger responsive version for an older website. Convert and resize. Not much else I can say here.
For more super cool plugins check out the blog by Vadim on thumblr: madebyvadim.tumblr.com