There are a ton here, even some for things that aren't here just yet:http://www.rosshaynes.com/WindowsOS.html
If I remember correctly, until XP you didn't have to connect to the internet or call Microsoft to activate Windows. Until Windows 95, product keys didn't really exist I don't think. Last I checked, 95 had a skip button too.
Only four I have on hand are:
Windows 98SE: PDFVF-9Q67V-FTYFT-4CJDV-GTBXB
Windows ME: All 1's.
Windows 2000 Professional: RM233-2PRQQ-FR4RH-JP89H-46QYB
Windows NT 4.0 Workstation: 30495-0006276-08164
I've got a copy of Windows 2000 "select" which doesn't require a product key, should I dump it or is there already one here?