There are three recommendations here: the first is Gadwin PrintScreen. It's a prime example of the KISS principle. It does exactly what I want, in the way I want, and it avoids the trap of providing a lot of useless and confusing features. It makes the hot key "PrtSc", and automatically names the output files in sequential order for multiple screen shots. It's these little things that count.
A second recommendation is Screen Hunter. It's a commercial product, but the "lite" free version is excellent and offers more features than Gadwin, albeit with of a little added complexity. It saves captures in .jpg, .bmp or .gif formats.
Also highly recommended is FastStone Screen Capture. It requires no installation, and yet provides more features than you might ever want, including the ability to capture scrolling screen shots across more than one screen page. It can also save in BMP, JPEG, JPEG2000, PNG, GIF, TIFF and TGA formats. In many ways it's like the full commercial version of ScreenHunter, except that it's free for personal use. NOTE: Since version 5.4, Screen capture has become commercial shareware. You can, however, get the last free version from here
Gadwin PrintScreen
Website: http://www.gadwin.com/products.htm
Download link: http://www.gadwin.com/download/
Author: Gadwin Systems
Current version: 4.3
Version date: August 20, 2007
License: Freeware
Download File size: 2826KB
Operating Systems Supported: Windows 98/Me/2000/2003/XP/Vista, Windows NT 4.0SP6
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None
Other relevant information: None
Screen Hunter Free
Website: http://www.wisdom-soft.com/products/screenhunter.htm
Download link: http://www.wisdom-soft.com/downloads/downloadfiles.htm
Author: Wisdom Software
Current version: 5.0.125
Version date: November 21, 2007
License: Freeware
Download file size: 3.3MB
Operating Systems Supported: 95/98/Me/NT/2000/XP/2003 Server/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None
Other relevant information: None
FastStone Screen Capture
Website: http://www.faststone.org/
Download link: http://www.faststone.org/FSCapturerDownload.htm
Author: FastStone Soft
Current version: 5.9
Version date: November 26, 2007
License: Shareware
Download file size: 1.4MB
Operating Systems Supported: 95/98/Me/NT/2000/XP/2003 Server/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: Yes
Non-English languages supported: None
Other relevant information: None
Fastone Screen Capture
Website: http://www.faststone.org/
Download link: http://www.aplusfreeware.com/categories/mmedia/FastStoneCapture.html
Author: FastStone Soft
Current version: 5.3
Version date: February 12, 2007
License: Free for home users
Download file size: 1380KB
Operating Systems Supported: 95/98/Me/NT/2000/XP/2003 Server/Vista
Additional Software Required: None
64 Bit Capable: No
Portable Version Available: No
Non-English languages supported: None
Other relevant information: None