Yahoo!® Messenger comes in a few flavours, Windows, Mac, Unix and it's also available as a Web Based messenger (no download required) but there is vast differences between the versions. Yahoo! Messenger for Windows offers all the features like animated smileys, webcam, voice, file transfer, stealth settings etc, the Mac version is sadly lacking the all but the bare basics that Windows users get although that has changed somewhat with the latest Mac beta. The Web Based version has, as expected, limited features, it also sometimes works and sometimes doesn't... things could be worse I guess but it is annoying at times :)

Current versions for Windows...
8.1.0.421 (for Windows 98 / 2000 / ME / XP / Vista) released 7 September 07
Homepage: http://messenger.yahoo.com/
Direct Download Location: (if you have installer issues)
http://us.dl1.yimg.com/download.yahoo.com/dl/msgr8/us/ymsgr810_421_us.exe
see our thread Yahoo! Messenger v8.1.0.421 for more information and compatible utilites :)

Basic System Requirements: 
Microsoft Windows 98, Windows 2000, Windows ME, Windows XP, Windows Vista
Microsoft Internet Explorer version 6 or later
Pentium II 233 Mhz or better (500MHz recommended)
Minimum 64 M B RAM for Windows 98 and ME
Minimum 128 M B RAM for Windows 2000 and XP
Minimum 30 Mb disk space
Windows Media Player - 7 or higher or higher
Webcam - Messenger compatible
A microphone, sound card, and speakers or a headset
more information can be found in our article Yahoo! Messenger v8 System Requirements

7.5.0.819 (for Windows 98 / 2000 / ME / XP) released 16 June 06
http://messenger.yahoo.com/
Help & Support: http://help.yahoo.com/l/us/yahoo/messenger/messenger75/index.html
Direct Download Location: (if you have installer issues) http://download.yahoo.com/dl/msgr75/us/ymsgr750_819_us.exe
more information available at WackyB.co.nz Yahoo v7.5

7.0.2.120 (for Windows 98 / 2000 / ME / XP) 23 December 05
Direct download: http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/us/ymsgr702_120_us.exe
Help & Support: http://help.yahoo.com/l/us/yahoo/messenger/messenger7/index.html
Basic System Requirements: 
Microsoft Windows 98/2000/ME/XP
Microsoft Internet Explorer v5.0+
Macromedia Flash Player 6.0+
Windows Media Player version 7+
56k or broadband Internet connection
microphone, sound card, and speakers
for more specific requirements and ways to avoid installation/upgrade issues check out our Yahoo v7 Installation, System Requirments, Settings & Paths
more information available at WackyB.co.nz Yahoo v7

6.0.0.1922 (for Windows 98 / 2000 / ME / XP) 27 Feburary 05
Direct download: http://download.yahoo.com/dl/msgr6/us/ymsgr6us.exe (thanks to DaveC and PaulK for this link :)
Help & Support: http://help.yahoo.com/help/us/messenger/win/index.html
Basic System Requirements: 
Windows 98/2000/Me/XP
Microsoft Internet Explorer - 5.5 or higher
Macromedia Flash Player - 6.0 or higher
Windows Media Player - 7 or higher
Webcam - Messenger compatible
DSP Group TrueSpeechTM audio compression (CODEC)
56k or broadband Internet connection (LAUNCHcast)
A microphone, sound card, and speakers
more information available at WackyB.co.nz Yahoo v6

5.6.1358 (for Windows 95 / 98 / 98SE / ME / NT / 2000 / XP ) 9 January 04
see Yahoo! Messenger Help Document: I can't run Messenger 6.0. How do I go back to Messenger 5.6?
more information available at WackyB.co.nz Yahoo v5.6

NOTES:
1. If you are looking for previous versions of Yahoo! Messenger that are still available for download directly from Yahoo! please check out this thread Yahoo! Messenger Versions Available at our forum which lists them all via country.
2. Recently Yahoo! dumped all previous version 6 and 5.6's off their server, a right royal pain in the bum if you're one who loves either of these or if they are the only versions your computer likes but all is not lost though because as at the time of writing this, while you are not able to download either of these versions from Yahoo! (directly) you can still connect to Yahoo's servers with them, the last version 5.6 was 5.6.1358 (9 January 04) which you can download from http://www.freecreed.com/ymsgr-archive.html along with numerous other previous versions :)

If you're running a previous version and you're finding the update notice annoying you can disable it by renaming yupdater.exe to yupdater.OLD, you can locate this file at C:\Program Files\Yahoo!\Messenger\

If you're having problems with the installer methods or wish to return to a previous version then try the direct download approach (like above)
e.g. say you want to get your hands on v7.0.0.437, use the following link direct link http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/us/ymsgr7_437_us.exe
if you wanted v7.0.0.426 use http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/us/ymsgr7_426_us.exe
so for past and future versions just update the link with the last 3 numbers of the version eg. 7.0.0.437 = ymsgr7_437_us.exe
International versions can be accessed the same way, the US version is located at
http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/us/msgr7us.exe
if you're after the German version you can grab it from
http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/de/ymsgr7_437_de.exe
and likewise for all other internationals, simply replace the local abbreviation
Australia & New Zealand (au) is http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/au/ymsgr7_437_au.exe
India (in) is http://us.dl1.yimg.com/download.yahoo.com/dl/msgr7/in/ymsgr7_437_in.exe
etc, etc

Current version for Mac...
Homepage: http://messenger.yahoo.com/mac.php / http://messenger.yahoo.com/mac.php
beta 1 r4 released 24 May 07
- http://download.yahoo.com/dl/mac/yahoo_messenger_3.0_beta1r4.dmg
beta 1 r3 released 17 May 07
- http://download.yahoo.com/dl/mac/yahoo_messenger_3.0_beta1r3.dmg
beta 1 r2 released 18 July 06
- http://download.yahoo.com/dl/mac/yahoo_messenger_3.0_beta1r2.dmg
beta 1 released 27 June 06
- http://download.yahoo.com/dl/mac/yahoo_messenger_3.0_beta1.dmg

2.5.3 - 18 Sept 2003
Basic System Requirements: 
A connection to the Internet
Mac OS 8.6 or newer
CarbonLib 1.5 (for Mac OS 9 and less)
Direct download:
OSX http://download.yahoo.com/dl/mac/ymsgr_2.5.3-osx_install.bin 1924kb
OS8/9 http://download.yahoo.com/dl/mac/ymsgr_2.5.3-ppc_install.bin 3565kb
As stated at the begining of this article, Yahoo! Messenger for Mac is sadly lacking... in fact it's perhaps just plain sad and then just happens to be lacking as well *cough* What's stated on their Mac homepage is very misleading with regard to the features and it tends to link to the Windows features not the ones actually available for the Mac version for more information about it's quirks check out our section for Yahoo (Mac) v2.5.3

Current version for Unix...
there is also a Unix version available from http://messenger.yahoo.com/unix.php, I don't know a thing about it, not a clue.. I don't use it, but feel free to let us know you're thoughts on it if you do :)

Yahoo! Web Messenger 
http://messenger.yahoo.com/webmsgr/fmsgr.php
Lets you talk to your Yahoo! friends ONLY
You need a Yahoo! Account, if you don't have one you can grab it from http://edit.yahoo.com/config/eval_register
Bare basic documentation is available at Yahoo! Web Messenger Help
Features:
Basic IM
System Requirements:
a browser that supports Java 1.1, JavaScript 1.2

Summary of Links:
Yahoo! Messenger Homepage: http://messenger.yahoo.com
Yahoo! Messenger for Mac: http://messenger.yahoo.com/mac.php
Yahoo! Web Messenger: http://messenger.yahoo.com/webmsgr/fmsgr.php
WackyB.co.nz Forum: http://www.wackyb.co.nz/vb/
WackyB.com Forum: http://www.wackyb.com
FreeCreed: http://www.freecreed.com