We start with the trusty, built-in Apple Color Picker, which is innately a great tool. It provides an almost universal interface for choosing and storing colors and has several handy methods of color choice and palette storage. I just needed a little extra power, so I went out seeking an upgrade. I eventually found Painter's Picker, a $15.85 utility which has been covered on TUAW before. With its ability to work in multiple color spaces, make use of a respectable amount of color theory and output a range of palette formats, it's a great tool for finding and tweaking colors and color combinations. There are plenty of Color Picker tools to choose from, including freebies, so take a look around. You'll find Mondrianum (a free Kuler interface we just mentioned), multiple flavors of free hex color pickers, even a tool to make the Color Picker handle spot colors (Spot Picker, about $30). And remember, you can swap out the Adobe color picker for the Apple one in Photoshop and make great use of the additional plugin power.
If you regularly work with colors, you probably collect a lot of swatches and palettes. You can put together collections in the Color Picker fairly easily, but it's a little cumbersome, and difficult to deal with crossing color spaces and switching profiles. If you're willing to drop some cash ($39.95), Tangerine is a tool that can handle all of that with ease, as well as integrate with the Color Picker and a slew of applications. When used as a standalone app (not inside the Color Picker) it can control the foreground, background, fill and stroke of various applications, in addition to inserting hex codes and even Objective-C and Carbon notations. It can hide in your menubar until you need it, and the floating palette can auto-hide on a per application basis.
Sometimes the palette you need is nowhere to be found, even with some great online resources, and you're drawing a blank. Well, with a little help from Genopal, you can usually get headed in the right direction. If you give it one color, you can randomly generate (mostly) pleasing, theory-based combinations and get some basic control over the output using simple sliders. I've found I can usually get what's in my head into a palette with a good base color and a few clicks of the randomize button. Genopal comes in two flavors: a standard version for $24.95 and a Pro version for $49.95. And yes, I agree that's a little pricey for what it does. Furthermore, the additional features in the Pro version are not, in my opinion, worth an additional $25, but the authors make the standard version very tricky to find. If you want to try it out, download the standard version here. If you like it, and think it's worth $25, use the internal link to buy it (you won't find it on the website).
So, there are a few highlights from the color portion of my toolbox. Some of them are a little redundant, or at least have overlapping features, so I'd recommend trying them to figure out which tools best suit your needs. I'm curious to hear in the comments what other people consider to be "must have" color tools, so please share!