rc2po [options] <rc> <po>
po2rc [options] -t <rc> <po> <rc>

Where:

AppleGlot

MacOS .strings file localization - Bug 371 relates to this.

Adobe:

FrameMaker's Maker Interchange Format - MIF (See also python-gendoc, and Perl MIF module)
FrameMaker's Maker Markup Language (MML)

Microsoft

Word, Excel, etc (probably through usage of OpenOffice.org)
OOXML (at least at the text level we don't have to deal with much of the mess inside OOXML). See also: Open XML SDK v1
Rich Text Format (RTF) see pyrtf-ng
XML Paper Specification
.NET Resource files (.resx) - Bug 396

XML related

Generic XML
DocBook (can be handled by KDE's xml2pot)
SVG
Android resource files specifically string resources

DITA

PDF see spec, PDFedit

LaTeX - see plasTeX, a Python framework for processing LaTeX documents

unoconv - Python bindings to OpenOffice.org UNO which could allow manipulation of all formats understood by OpenOffice.org.

Trados:

TTX (Reverse Engineered DTD, other discussion)
Multiterm XML TSV to MiltiTerm conversion script

Tcl: .msg files. Good documentation

Installers:

NSIS installer: Existing C++ implementation
WiX - MSI (Microsoft Installer) creator. Localization instructions, more notes on localisation. This is a custom XML format, another one!

gencat: precedes gettext, looking in man packages is the best information I could find. Also LSB requires it.

Unlikely to be supported

These formats are either: too difficult to implement, undocumented, can be processed using some intermediate format or used by too few people to justify the effort. Or some combination or these issues.

rc2po

Usage

Unlikely to be supported