Image_Freeware

All these programs work on Windows XP systems. Most will work on earlier or later Windows versions. A few have versions that work on Linux and other operating systems.

WARNING: Always work on copies, not originals, so that when you make a mistake you don't permanently ruin your photo. Some of the programs allow you to put the edited image in another folder rather than replace the unedited file - use this option.

CAUTION: Most of these programs will offer to be the default handler for many image file formats (by default unless un-selected). Make sure you un-select these options unless you really want to change your system defaults.

Google Picasa
Picasa will search your computer for images, so one of the first things to do is to limit it to looking where you keep your photographs. The editing capabilities are rather basic. It is most useful for such tasks as making CD slideshows, uploading to a web album or interfacing with Google Earth.
Picasa Home Page

IrfanView
With the plugins installed this program will allow you to view just about any image file, and convert to another format. Video and audio files can also be played (I use it as default for MP3 and WAV files as it starts much faster than Windows Media Player). Batch conversion (with editing in the advanced settings) is a strong point. It also makes a good viewer for browsing a set of images. The Paint dialog is nice for annotations. It can display EXIF and edit IPTC information and link to Google Earth. The MANY configuration options make it a little difficult to set up, so my INI File is available as a starting point. Save it in the folder with the program.
IrfanView Home Page

Faststone Image Viewer
Similar to IrfanView, it is integrated into a single window which makes it a bit easier to use. It is very good for cropping and annotating images. The batch conversion with advanced options is similar to IrfanView. The browsing function is easier than IrfanView.
Faststone Home Page

XNView
Another versatile image viewer, with similar batch processing, however the batch renaming is far more powerful. It can also display EXIF data and edit IPTC information. It can be used to browse, but Faststone is easier for this. The histograms display gives you an idea of exposure balance.
XNView Home Page

Photoscape
Again, similar to the previous programs with the addition of a basic RAW conversion, animated GIF, a screen capture function and page layout for printing.
Photoscape Home Page

As they are all free and small (except Picasa) load them all and use whichever is more suited to you for particular tasks. If I had to choose only one, I'd select XNView.

The GIMP
This is just about equivalent to the commercial Photoshop. Originating in Linux world, it is well supported and constantly evolving. This is a bit of a problem as the help and tutorials can't keep up. Like Photoshop it is extensible with plugins. The tutorial "Grokking the GIMP" is well written, but a few versions behind, however it is a good reference (the techniques are good, but the menu system in the GIMP has changed a bit).
GIMP for Windows download Get the Help file too.
Grokking the GIMP

Paint.NET
This is a newer entry in the field, but rapidly catching up with others in power. It needs the .NET framework from Microsoft.
Paint.NET Home Page

Google Earth
Not strictly an image viewer, but if the location of your photo is recorded in the EXIF data (eg by Picasa) then Picasa can save a file which displays your photos in Google Earth.
Google Earth Home Page

ExifTool
A program (command line or Perl script) to read and write EXIF and IPTC data in your image. A GUI shell is available to help in common tasks.
Exiftool Home Page
Exiftool GUI Home Page

ImgSearch
Find duplicate or similar images that clutter your system. Variable sensitivity allows degree of similarity to be set.
ImgSearch Download

Although the simple image viewers can convert RAW to JPG, that's about all they do. The following handle RAW files well ---

Raw Therapee
A stand-alone processor.
Raw Therapee Home Page

UFRaw
A front-end for The GIMP.
UFRaw Home Page

CHDK
This doesn't run on your computer, but on your Canon camera. It adds functions found on high-end cameras to those with lower capabilities. It can show properties to help you shoot better pictures (eg depth of field, histograms), can extend exposure times, save images in RAW format and show storage and battery capacity. There are scripts to perform time-lapse and motion detection. Plus a lot more.
CHDK Wiki Page