Registration is now mandatory on Freenode :
Fill in a nickname, click "I'm not a robot", then, "Connect"
Type these lines in the bottom text field:
/nick mypseudonym (the name you want)
/msg NickServ REGISTER password email@example.com
You will get an email with the third line to paste in.
That's it, you now own 'mypseudonym' on Freenode.
Keep that password!