Since "resources" can be interpreted many ways, I'll just do a raw dump of the best shit I can think of
Linux OS:
http://it-ebooks.info/book/2014/Free PDF, 1400 pages. Hands down the best linux book I've come across. Stuff that has taken me years to learn on my own, just running across through advice from professionals, I read in here as well.
very well known, but great if you haven't it:
http://www.exploit-db.com/(find exploits)
the hacking toolbox. just by installing this and reading up what each tool does will teach you a shit load:
http://www.kali.org/the book greyhat python. not free I don't think, but a great resource.
Practical Packet Analysis - no starch press I think. This is where I learned most of my wiresharking. Still, learn tcpdump. Nothing makes up for tcpdump.
best tools: netcat, tcpdump, python + scapy, metasploit, SET, Browser Exploitation Framework (BeEF), hexdump, objdump, radare2, gdb
best netsec online mag:
http://www.phrack.org/Find ANY and ALL CTF write-ups. They can teach you a lot. Some will be obscure tricky problems but some will actually teach you cool stuff. Check out defcon CTF writeups.
I hear yara is great for IDS rules:
http://plusvic.github.io/yara/don't hate on reddit. It can be a great resource. Check out /r/netsec , /r/crypto , /r/malware /r/xss /r/reverseengineering and even /r/blackhat has something now and then.