There are lots of text editors. Some of these aspire to be Notepad replacements while others are full-on programming editors.
My top choice in the first category is EditPad Lite [1]. It has a Notepad-like interface combined with tabbed document windows, the ability to open as many documents as you like, no file size limitations and unlimited un-do capability. It's main downside is that it's for personal use only.
If you need a free Notepad replacement that you can use commercially I suggest NotePad2 [2]. It's small and fast, but unfortunately lacks the tabbed Windows that are so handy in EditPad. NoteTab Lite [3] is another possibility but personally I find it a tad slow and the single-level undo a severe limitation.
If, however, you are looking for a text editor that can serve both as a Notepad replacement and a source editor you might like to check out Notepad++ [4]. It performs impressively in both roles. It's loaded with features to make your programming more productive including syntax and brace highlighting for many languages, search and replace using regular expressions, macro recording and more. It is also highly configurable through plug-ins, has a wide range of themes and offers multi-language support.
More specifically geared to programming is PSPad [5]. It supports syntax high-lighting for most popular languages, has an inbuilt spell checker, hex editor, macro recorder, FTP client and more.
Other source code editors worthy of evaluation are ConTEXT [6] and my personal favorite, Crimson [7].
Choosing the "best" programming editor is too controversial a topic even for me so I suggest you try all these and see what works best for you.
[1] EditPad Lite
Website: http://www.editpadpro.com/editpadlite.html
Download link: http://www.editpadpro.com/editpadlite.html
Author: Just Great Software
Current version: 6.3.1
Version date: August 16, 2007
Download file size: 3.1MB
License: Free for non-commercial use
Operating systems supported: Windows NT4/98/2000/ME/XP/Vista
Additional software required: None
64 Bit version available: No
Portable version available: Yes
Non-English languages supported: Yes
Other relevant information: No
[2] NotePad2
Website: http://www.flos-freeware.ch/notepad2.html
Download link: http://www.flos-freeware.ch/notepad2.html
Author: Florian Balmer
Current version: 2.0.18
Version date: July 26, 2007
Download file size: 247KB
License: BSD
Operating systems supported: NT-based versions of Windows
Additional software required: None
64 Bit version available: No
Portable version available: Yes
Non-English languages supported: Yes
Other relevant information: No
[3] NoteTab Lite
Website: http://www.notetab.com/ntl.php
Download link: http://www.notetab.com/ntl.php
Author: Fookes Software
Current version: 5.6
Version date: January, 2008
Download file size: 2.08MB
License: Freeware
Operating systems supported: Windows 95/98/2000/2003/NT4/ME/XP/XP64/Vista
Additional software required: None
64 Bit version available: Yes
Portable version available: Yes
Non-English languages supported: None
Other relevant information: Many free resources for NoteTab Lite are available on the website.
[4] Notepad++
Website: http://notepad-plus.sourceforge.net/uk/site.htm
Download link: http://sourceforge.net/project/showfiles.php?group_id=95717&package_id=102072
Author: SourceForge
Current version: 4.7.5
Version date: January 14, 2008
Download file size: 1.99MB
License: GNU General Public License
Operating systems supported: Windows 95/98/ME/NT/2000/2003/XP and some versions of Ubuntu Linux
Additional software required: No
64 Bit version available: None
Portable version available: Yes
Non-English languages supported: Yes
Other relevant information: A lot of plug-ins are available on the website
[5] PSPad
Website: http://www.pspad.com/
Download link: http://www.pspad.com/en/download.php
Author: Jan Fiala
Current version: 4.5.3 (Build 2298)
Version date: November 25, 2007
Download file size: 3.43MB
License: Freeware
Operating systems supported: Windows 98/2000/ME/NT/XP/Vista
Additional software required: None
64 Bit version available: No
Portable version available: Yes
Non-English languages supported: Yes
Other relevant information: 3 pages of editor extensions are available on the website
[6] ConTEXT
Website: http://www.context.cx/
Download link: http://www.context.cx/downloads.html
Author: ConTEXT Project Ltd
Current version: 0.98.5
Version date: December 3, 2006
Download file size: 1.6MB
License: Freeware
Operating systems supported: Windows 95/98/ME/NT/2000/XP/Vista
Additional software required: None
64 Bit version available: No
Portable version available: No
Non-English languages supported: Yes
Other relevant information:
[7] Crimson
Website: http://www.crimsoneditor.com/
Download link: http://www.crimsoneditor.com/
Author: Ingyu Kang
Current version: 3.70
Version date: September 22, 2004
Download file size: 1.19MB
License: Freeware
Operating systems supported: Windows 95/98/ME/NT/2000/XP
Additional software required: None
64 Bit version available: No
Portable version available: No
Non-English languages supported: Yes
Other relevant information: Crimson Editor became an open source project in December 2006. It is being rewritten under the name Emerald Editor. There is Beta version 3.72 information on their website http://www.emeraldeditor.com/