I've created a content type on my local dev server, how can I package this content type, so I can deploy it professionally to live installations?
I would suggest you to look at the WSP Builder. It will help you a lot when creating SharePoint WSP solutions.
An option is AC's WCM Custom Commands for STSADM.EXE which adds two commands which is relevant here:
- GenSiteColumnsXml
- GenContentTypesXml
These allow you to get the XML for Site columns and content types base on ones existing in a site
SPSource is an open source project that can reverse-engineer quite a few SharePoint artifacts and is actively supported by knowledgeable SharePointers such as Jeremy Thake and Rich Finn. There is a bit of set-up time required however.
There's also SharePoint Solution Generator from the Visual Studio Extensions for WSS (although I would use SPSource over this any day).
Note that both of these options are much more comprehensive than what you've asked for and designed to extract the artifacts from an entire SharePoint site. So if you just want the content types go with Per Jakobsen's answer.