http://poptop.lineo.com/  
PoPToP is a PPTP Server for Linux

http://sourceforge.net/projects/l2tpd/
A Layer 2 Tunneling Protocol VPN client/daemon for Linux and other POSIX-based OSs

http://www.interpeak.com/products/l2tp.html
L2tp client for embedded apps ?

http://slashdot.org/article.pl?sid=00/10/04/154239&mode=thread
discussion about l2tp & masquerading in linux.

http://www.townsley.net/
This guy is on the RFC for L2TP

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t1/l2tpt.htm
Huge doc from Cisco, covers everything about l2tp.

http://www.cisco.com/warp/public/707/24.html
Another doc, this one for configuring Cisco's to act as an LAC or LNS

http://www.ietf.org/html.charters/l2tpext-charter.html
Something New - Layer Two Tunneling Protocol Extensions (l2tpext)