TheoraSoftwareEncoders

Multi-platform

ffmpeg2theora a commandline encoder from any format read by ffmpeg to Theora/Vorbis: svn, latest releases.
VLC Media Player Can transcode from any source it supports into Ogg/Theora. WARNING: Apparently creates broken Ogg streams.
GStreamer GStreamer is a library that allows the construction of graphs of media-handling components, ranging from simple Vorbis playback to complex audio (mixing) and video (non-linear editing) processing.

Visonair.tv Ogg Streamer A Windows application to stream directly from a webcam to an Icecast server.
Visonair.tv Virtual Stage Includes an application to encode to Theora, forces fixed size and encoding parameters though. Registration required.
GFrontend ffmpeg2theora GUI Frontend for ffmpeg2theora
MediaCoder Application to encode media files into many target formats, including Theora.
SUPER General purpose converter application, also serves as a frontend to ffmpeg2theora.

Thoggen is a DVD backup utility ('DVD ripper') for Linux, based on GStreamer and Gtk+.
Freej Freej is a realtime video mixer. It can stream Theora and Vorbis live to icecast. Check here for more info.
OggConvert is a small Gnome utility which uses GStreamer to convert (almost) any media file to Vorbis, Theora and Dirac.

XiphQT allows you to export from Quicktime, or any application supporting Quicktime (i.e. Final Cut Pro), to Theora/Vorbis.
Simple Theora Encoder an ffmpeg2theora frontend