This article is from the GNU Chess and XBoard FAQ, by Tim Mann email@example.com with numerous contributions by others.
XBoard and WinBoard have simple but effective bughouse support. Offboard
piece holdings are shown in the board window's banner, and you drop
offboard pieces using the right mouse button. Press it over the
destination square to pop up a menu of pieces.
XBoard and WinBoard can display only one board at a time, but you can
observe your partner's game by running a second copy of the program and
logging in as a guest. (Unfortunately, this is not possible if you are
using the /icscomm option.) To observe your partner's games
automatically, use the "follow" or "pfollow" ICS command; see the ICS
online help for details.