Naldz Graphics has a list of Photoshop actions to enhance photos. I use the Lightroom route to process photos, but some actions can be good to have in Photoshop for inspiration or just playing around.
Archived entries for photoshop
Carsonified has an interesting artocle with some photoshop tutorials on design elements for the web with Photoshop.
Smashing Magazine has a great article on the Pen tool. I haven't gone through it yet, but the pen tool is one of the most useful and misunderstood tools to use. Lots of helpful info in this.
Smashing Magazine has a great article about how to tweak Photoshop to get better performance:
Even though it's now becoming outdated, sixrevisions.com have a good article on how to create a web 2.0 look in Photoshop using the 960 grid system. It's a two part article that will include how to code it in CSS.
Designm.ag has a good post listing over 20 photoshop actions.
SEAMonster is a freeware image resizing and cropping tool that uses seam carving. Seam Carving is basically a way of keeping important parts of images intact and removing other parts that have little difference in the pixels (e.g., the sea).SEAMonster can be downloaded here.
I haven't used one before, but it reminds me of another Photoshop plugin called Genuine Fractals. This uses an algorithm to resize images with minimum pixelation. It basically uses a best guess algorithm to fill in pixels of the resized image. This is used in contrast to using bicubic resizing, which resizes an image but duplicates it's neighbouring pixels attributes.