iBus is described as 'Intelligent Input Bus'. There are three alternatives to iBus for Linux, BSD and Mac. The best alternative is UIM, which is both free and Open Source. Other great apps like iBus are fcitx (Free, Open Source) and scim (Free, Open Source).
Fcitx ['fa?t?ks] is a input method framework with extension support. Currently it supports linux and Unix system, like freebsd. It has three builtin Input Method Engine, Pinyin, QuWei and Table-based input methods.
The Smart Common Input Method (SCIM) platform project provides not only a user friendly, full featured input method user interface for POSIX-style operating systems (including Linux, FreeBSD and other Unix), but also a development platform to make input method development easier.