One of the first things I look at when a client asks me "why is my web site so slow" is the admin/build/modules page (actually, the first thing I look at is the client's internet connection). It's very rare that I don't find more than a couple of modules that are enabled for absolutely no reason - sucking down precious server resources.

Here's a few that I find most often - and are usually no-brainers to disable:

• Content Copy - this module, part of CCK is only necessary when you're importing or exporting content types. Very useful during development, but almost never necessary on a production site.
• Color - unless your theme is Color-module enabled (Does your theme's settings have a color wheel that allows you to change the color of your site? No? Then your theme isn't Color-module enabled), disable it.
• Help - nice to know that it's there, but how many people do you know that actually use it? Check with your client first, but 9 times out of 10, they don't use it.
• ImageCache UI - Once the various ImageCache presets are setup, you can safely disable this module and gain back some additional computing cycles.

Trust me, this list is far from complete - go take a look at your modules page and you'll be surprised at what you're not using.