emx 0.9a supports sockets without the Programmer's Toolkit. The socket support of emx is `seamless': fdopen(), for instance, can be used on sockets.
IBM's TCP/IP 2.0 ($200) includes an optional Programmer's Toolkit ($500, part #02G6973). It includes a socket library, and support for Sun RPC, NCS RPC, and a limited Kerberos capability. It requires IBM C-Set++ or another compiler that understands 16-bit code.
For REXX programs there is a socket library called REXXSOCK available from cdrom.com as os2/ibm/ews/rxsock.zip. It can be used to write TCP/IP applications such as Gopher, IRC, etc.
FTP Software, Inc., has an OS/2 version of its TCP/IP product. They can be reached at (617) 246-0900 or info@ftp.com.
If you have the IBM TCP/IP 2.0 base package and IBM C-Set++, you can use the TCPIPDLL.DLL directly.