For easy work with IBM AS/400 sessions, try this TN5250 emulator.
Telnet (SSL/TLS/SSH) terminal (3270/5250/VT/ANSI) emulator.
For easy work with IBM Mainframe sessions, try the popular TN3270 emulator.
Mocha TN3270 provides TN3270 emulation for IBM Mainframe Access.
It is a terminal program for Telnet (TCP/IP), ISDN (CAPI) and modems (RS232).