aPLib - http://www.ibsensoftware.com
Without aPLib, Packman would not be possible unless I actually got off my butt and wrote my own data compression algorithm. What makes aPLib great is its fast and small decompression routine.
LordPE - http://y0da.cjb.net
Perhaps the best PE editor out there.
LZMA - http://www.7-zip.org
Great compression library for compressing large amounts of data.
hutch's masm forum - http://www.masmforum.com
Great forum for getting help with assembler releated topics. Friendly automosphere and expert advice.
Matt Pietrek's site - http://www.wheaty.net
Check out some of his books and columns for some in depth knowledge of the Windows operating system.
MSDN - http://msdn.microsoft.com
Provides very detailed information about the Windows API, which is of course essential for Windows Programming.
Programmers Tools - http://programmerstools.org
Great site for finding more executable packers and much more.
Resource Hacker - http://www.angusj.com/resourcehacker
Resource editor that is a lot more detailed than LordPE's resource viewer.
Reverse-Engeneering-Academy - http://www.reverser-course.de
Reversing knowledge was essential to writing and understanding this program. Packman is a small example of GOOD use of these scorned topics. Unfortunately new registration is unavailable right now, but hopefully this will change soon. When it does, I challenge you to sign up and give it a go if you are interested in greatly improving your programming skills.