For a lot more, see IPTC's list of Software supporting IPTC photo metadata standards IIM and IPTC Core
ACDSee Pro - can read/write XMP information for DNG, GIF, JPEG, PNG and TIFF files.
Adobe applications (Photoshop, Bridge, etc.).
Daminion - Image, Video and Document Management Solution for Creative Professional - read/write tags from/to XMP, IPTC, EXIF to most image formats, including camera RAW and vector graphics as well as most video and audio formats.
DBGallery - A digital photo system primary dealing with photo data which reads/writes/edits IPTC/XMP data to a large variety of image formats (PSD, DNG, JPEG, most RAW formats, PNG, TIFF, plus others). (Windows, .Net 2.0).
EMET (Embedded Metadata Extraction Tool) - Adobe Air 2.0-based tool designed to extract metadata embedded in JPEG and TIFF files.
Exempi - Open Source library for manipulating XMP, built on the Adobe XMP SDK.
ExifTool (complete reader/editor for EXIF, IPTC, and XMP).
ExifToolGUI (Windows GUI) Used with ExifTool, this is the best free metadata viewer I have found. It also allows you to import and export embedded metadata in a single image or in batches
Exiv2 - Image metadata library and tools.
FastPictureViewer - Image viewer (Windows) with XMP embedding and/or sidecar files creation (xmp:Rating, xmp:Label, photoshop:Urgency).
Flickr Tools - Flickr retains embedded metadata and allows you to download it. These tools allow bulk downloads.
Flickr Edit (Windows) - FREE (upload/download photos and EXIF and IPTC metadata metadata) .
Downloadr (Windows) - FREE (download photos and EXIF and IPTC metadata) .
Bulkr (Windows, Mac) - FREE for basic (download photos), $24.95 for pro (download photos and EXIF and IPTC metadata. Also exports .txt file of metadata!) .
FotoStation Pro - commercial digital asset management.
Graphic Converter for Mac $35/$45 (editor and organizer, edits Exif, IPTC and XMP. SUpports ExifTool).
IDimager - customizable xmp schema.
iTag - FREE - basic IPTC/XMP data.
IPTC - The ubiquitous embedded metadata standard.
IPTC-PLUS Photo Metadata Toolkit for Adobe CS - CS3/4 Java plugin and CS5 user guide.
Applications that support IPTC.
iView Media Pro (pro photo digital asset management).
Kalimages - an innovative IPTC and XMP retrieval and editing software for image files allowing to catalog metadata into an integrated database to rapidly review, index, identify and view photos).
Media Bin - Enterprise commercial digital asset management.
FileMind - enables you to review, edit and export any file-based metadata in a single place.
Wiki for FileMind and the MetaMind Framework (MMF) documentation.
Blog by developer Hans Fremuth.
MetadataTouch™ a powerful tool to view and edit metadata in Microsoft Office® 2007 documents (Word, Excel®, and PowerPoint®), OpenOffice.org / StarOffice™ documents, AVI video files, WAV audio files, JPEG image files, PNG image files, SVG image files, and XMP files.
Microsoft Expression Media 2 (Media database and workflow tool - add, edit, or export metadata in IPTC, XMP or Dublin Core).
Microsoft Windows Imaging Component (Windows Vista & Windows Vista x64 Edition to enable XMP metadata with complex data types to be stored using Windows Imaging Component)
Microsoft Windows Live Photo Gallery (writes tags to XMP)
National Library of New Zealand (Introduction to Metadata)
This open-source software is designed to automatically extract metadata from digital files, and output that metadata in XML so it can be loaded into a preservation metadata repository.
Open Text Media Management (formerly Artesia) - Enterprise Digital Asset Management application, supports XMP and IPTC.
PicaJet - digital photo and video management software with multi-user access. Supports XMP, IPTC, EXIF
RoboFolder - manages your photos by re-arranging the image folder structure to logical folder structure you can understand.
RoboImport - downloads your digital images straight from a digital memory card or a digital camera, renames them, and automatically perform's a number of image editing operations during transferring to your PC.
Python XMP Toolkit (C/C++ XMP library based on Adobe XMP Toolkit)
Xinet WebNative Venture - DAM (Digital Asset Management) system that allows read/write of XMP across all supported file types.
XnView (utility for viewing and converting graphic files)