A lot of posts in the fr and us newsgroups talks about customization.

Here is a list of web resources to know, that helps considerably during your customization process :

• http://msdn.microsoft.com/library/en-us/odc_SP2003_ta/html/ODC_SPSCustomizingSharePointSites1.asp : customize SharePoint sites and areas; manage users, sites, and templates; change the structure of pages in a site; manage user access; and optimize style sheets
• http://msdn.microsoft.com/library/en-us/odc_sp2003_ta/html/ODC_SPSCustomizingSharePointSites2.asp : site templates and definitions
• http://msdn.microsoft.com/library/default.asp?url=/library/en-us/odc_SP2003_ta/html/ODC_SPSCustomizingSharePointSites3.asp : Stylesheet reference table (this one is very useful according to the stylesheets length and complexity)
• http://www.sharepointcustomization.com/default.aspx : Customization kit for SharePoint products
• http://wss.collutions.com/pages/IV%20-%20Basic%20Customization.aspx : Mike Walsh's contribution to customization

If I find more, or if you have more, I'll update this list.