one of the first and foremost tool/software I prefer is IrfanView. you can download it from the website http://www.irfanview.com/ . This is one of the good starting point. This is a photo viewer tool with some(or lots of ) features to edit photos. This is one of the best tool to add border to photos very fast. You can edit the color contrast/hue/saturation etc.
GIMP : "GNU Image Manipulation Program". My most favorite image editing tool. Now a days, I use only GIMP for any kind of photo editing. Visit http://www.gimp.org . An amazing tool..
Adobe Photoshop CS3: A marvelous tool developed by Adobe. One of the great tools available for photo editing.
In the photo editors communities you can find lot of debates happening on Which is best tool in GIMP and Adobe. :)
There are lots of softwares... I will update this thread slowly... You also can contribute to this one... Please comment the details of softwares.. I will add that also to this...