There’s countless modules for Drupal that are underrated, but I wanted to call attention to three that I use regularly. These modules can make a big difference when building a theme, and while there’s a learning curve associated with some (Color module, I’m looking at you), they’re all worth the effort.
There’s several options to create headers with non-web-standard fonts (sIFR and Dynamic Rendering), but Signwriter is a very handy theme module. Basically, it takes a string of text, and generates an image based on profiles (which are very easy to override or create programatically) and the True Type or Open Type font file that you supply. The end result is a nifty image header, but can be used for all sorts of artistic effects anywhere on the site.
Anyone familiar with the Garland theme knows that you can change the color scheme. But many people don’t know that that effect is not limited to Garland. Any theme, if built using a sprite image for CSS images, can be color-ized. I once built the theme for a monthly magazine (Skirt.com) who wanted a different color scheme every month to match a featured piece of art; what would have been a nightmarish change to theme code and images every month became a simple task of choosing colors from a color wheel. The site has since redesigned (and no longer changes colors), but it’s still a very cool module. There’s even a Color.module: Next Generation module in current development that looks cool.
Color is part of Drupal core
Imagecache is one of those modules that’s so helpful to building a site, however the one thing it has trouble with is user avatars. A user’s picture is handled differently than most other images, and so when a user updates their profile, images created by Imagecache may be unaffected; Imagecache Profiles fixes that problem for you.