I'm looking for some way to capture an image of a PC after I run a sysprep - a lot like WDS but without networking on the PC. So instead of the network part it would be possible to (with an easy wizard like on WDS) create an image of the PC to a USB drive or something similar.
Clonezilla has always worked very well for me, check it out Clonezilla.org
The Microsoft Deployment Toolkit is an officially supported method for doing what you want to do. If hadn't created the image already, it would help you do that. It will also help you put the image onto a CD/DVD, USB drive, or WDS server.
I should probably add that this isn't technically a WDS alternative. It works with WDS if you choose. WDS is just one option for deploying an image. MDT is like a suite of tools that help create images and give you more options around how to deploy.
The tool you want is called ImageX and is part of the Windows Automated Installation Kit (WAIK). You'll need to create bootable media (DVD, USB Flash Drive, etc) with a WinPE boot image on it - again all the resources to do this are in the WAIK. Sysprep your reference PC and have it shut down. Then boot from the WinPE media, run ImageX to image the hard drive. ImageX produces a Windows Image (.WIM) file.
ImageX is a command-line tool that enables original equipment manufacturers (OEMs) and corporations to capture, modify, and apply file-based disk images for rapid deployment. ImageX works with Windows® image (.wim) files for copying to a network, or it can work with other technologies that use .wim images, such as Windows Setup, Windows Deployment Services (Windows DS), and the System Management Server (SMS) Operating System Feature Deployment Pack.
You can then use that WIM file with WDS, or create bootable media with it.