nLite. Lets you put drivers, service packs, individual updates etc in there. If you're building custom PCs it's definately the most professional way about it, as I hate using Ghost and I hate even more when people send you recovery disks with a ton of software that you never need. Just fills up the registry for the fun of it.
Also, if you're clever and the software you're installing has a silent install mode you can incorporate it... I'm thinking Office 2003 and Transform files, or whatever 2007 calls them now with it's custom install wizard, there's things like BACS for Broadcom NICs which I know do silent installs...