Some (but not all) Open Source products have a numbering system where the second number is even for a production release and odd for a development release - so if you're looking to install one of these products on a live machine for customer use, you should make sure you stick with the even release even if a later odd one is available.

Products that currently use this system include:
The Linux Kernel (2.2, 2.4 now at 2.6) [learn]
Perl (5,6, 5.8, now at 5.10) [learn fundamentals] [learn advanced] [learn for web]
Apache httpd (2.0, now at 2.2) [learn]

Open Source software has an active developer community, and the mechanism requires that there is considerable exposure given to upcoming releases for things to be added, comments made, and code tested and improved by that community, with anyone welcome to play his / her role. For this reason, you'll see a wide variety of extra releases on all open source software that's almost completely absent from commercial software. Commercial software's upcoming releases need to be kept rather private to give the authors their commercial edge, and also to ensure that orders for the current release don't dry up when potential customers say "I won't spend my money on this version - I'll buy the next one when it comes out".

You may still see Apache httpd 1.3, and Perl 5.004 and 5.005, around from time to time. In each of these cases the odd number release was a production version prior to the adoption of the "odd/even" scheme. Apache httpd 1.3 in particular is now extremely mature, but also very commonly used indeed as people don't like to upgrade a web server that's working perfectly well. "If it ain't bust, don't fix it"
(written 2008-11-21, updated 2008-11-24)

 P050 - Perl - General
  [116] The next generation of programmer - (2004-11-13)
  [400] New in the shops - (2005-08-01)
  [743] How to debug a Perl program - (2006-06-04)
  [1750] Glorious (?) 12th August - what a Pe(a)rl! - (2008-08-12)
  [2228] Where do I start when writing a program? - (2009-06-11)
  [2242] So what is this thing called Perl that I keep harping on about? - (2009-06-15)
  [2374] Lead characters on Perl variable names - (2009-08-24)
  [2504] Learning to program in ... - (2009-11-15)
  [2736] Perl Course FAQ - (2010-04-23)
  [2783] The Perl Survey - (2010-05-27)
  [2825] Perl course - is it tailored to Linux, or Microsoft Windows? - (2010-06-25)
  [2971] Should the public sector compete with businesses? and other deep questions - (2010-09-26)
  [3093] How many toilet rolls - hotel inventory and useage - (2010-12-18)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3407] Perl - a quick reminder and revision. Test yourself! - (2011-08-26)
  [3823] Know Python or PHP? Want to learn Perl too? - (2012-07-31)
  [3902] Shell - Grep - Sed - Awk - Perl - Python - which to use when? - (2012-10-22)
  [3911] How well do you know Perl and / or Python? - (2012-11-04)
  [4296] Polishing the Perl courses - updated training - (2014-09-17)
  [4301] Perl - still a very effective language indeed for extracting and reporting - (2014-09-20)

A601 - Web Application Deployment - Apache httpd - an overview
  [576] Why run two different web servers - (2006-01-25)
  [659] Web Application Components - (2006-03-28)
  [924] The LAMP Cookbook - Linux, Apache, MySQL, PHP / Perl - (2006-11-13)
  [1265] Apache, Tomcat, Jakarta, httpd, web server - what are they? - (2007-07-13)
  [1593] Keep the client experience easy - single server contact point - (2008-03-27)
  [2016] Apache httpd and Apache Tomcat miscellany - (2009-01-30)
  [2038] Sticky Sessions with mod_jk (httpd to Tomcat) - (2009-02-12)
  [2054] Tuning httpd / the supermarket checkout comparison - (2009-02-26)
  [2063] Internal Dummy Connections on Apache httpd - (2009-03-02)
  [2077] Why put Apache httpd in front of Apache Tomcat - (2009-03-12)
  [2186] An FAQ on the Apache httpd and Apache Tomcat web servers, and on using them together - (2009-05-17)
  [4064] Apache httpd - a robust, open source web server - (2013-04-16)
  [4434] Public training courses - upcoming dates - (2015-02-21)

A101 - Web Application Deployment - Linux -An Introduction For Users
  [73] vi - full circle - (2004-10-04)
  [74] pushd and popd - (2004-10-05)
  [152] Aladdin, or careful what you wish. - (2004-12-15)
  [249] An easy way out - (2005-03-17)
  [430] Linux commands - some basics - (2005-08-31)
  [431] File permissions of Linux and Unix systems - (2005-08-31)
  [593] Finding where the disc space has gone - (2006-02-06)
  [679] More or less on the edge of the page - (2006-04-11)
  [703] Copying files and preserving ownership - (2006-04-28)
  [710] Linux training Glasgow, Python programming course Dundee - (2006-05-05)
  [711] THE home directory or MY home directory - (2006-05-06)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [984] Cardinal numbers and magic numbers - (2006-12-14)
  [1012] Moving files between Windows / DOS and Linux / Unix - (2006-12-30)
  [1013] Copy multiple files - confusing error message from cp - (2006-12-30)
  [1068] ls -l report, Linux / Unix - types and permssions - (2007-02-06)
  [1259] Where am I and how did I get here? - (2007-07-05)
  [1287] Work and play at Well House Manor - Football and Shell Shortcuts - (2007-08-02)
  [1288] Linux run states, shell special commands, and directory structures - (2007-08-03)
  [1366] awk - a powerful data extraction and manipulation tool - (2007-09-25)
  [1408] Wireless hotel tips - FTP and Skype connections failing - (2007-10-26)
  [1438] Copy and paste / cut and paste and other vi techniques - (2007-11-20)
  [1527] Selecting file names in a shell - one word or another - (2008-02-02)
  [1651] ls command - favourite options - (2008-05-23)
  [1764] Yank and Push - copy and move in vi - (2008-08-21)
  [1803] FTP passive mode - a sometimes cure for upload hangs - (2008-09-20)
  [1893] Some Linux and Unix tips - (2008-11-18)
  [1902] sstrwxrwxrwx - Unix and Linux file permissions - (2008-11-23)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2201] Running straight from the jar, but not from a tar - (2009-05-26)
  [2203] Always use su with minus. And where do programs come from? - (2009-05-27)
  [2299] How much space does my directory take - Linux - (2009-07-20)
  [2300] What does x on a linux directory mean? - (2009-07-21)
  [2479] Accidentally typed ci rather than vi? - (2009-10-27)
  [2494] Making Linux Politically correct - (2009-11-06)
  [2636] Linux - useful tips including history and file name completion - (2010-02-15)
  [2831] Recording (a macro) in vi - (2010-06-27)
  [3179] Oops - I typed ci not vi, and have lost my file ... - (2011-02-21)
  [3256] Displaying a directory or file system tree - Linux - (2011-04-22)
  [3791] The Kernel, Shells and Daemons. Greek Gods in computing - (2012-07-01)
  [3819] Packing a tar, jar or war file - best practise - (2012-07-26)

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