One of the trickest parts of web development is ensuring your code is backwards compatible, there are a variety of utilities that can be utilised in order to assist with this, below I’ve listed the most useful ones that I have found.
Internet Explorer Application Compatibility VPC Image
The most useful of these utilities is provided by Microsoft, realising that a lot of people were still using older versions of Internet Explorer they release Virtual PC (external link) images containing versions of Internet Explorer from version 6 onwards. The images expire after a few months but Microsoft are constantly updating them to include the latest Windows updates.
Download the Internet Explorer Application Compatibility VPC Images (external link) from Microsoft’s site.
Browsershots (external link) is a open source web farm running multiple versions of operating system and browsers which can be used to check rendering of a single web page. As it is free it can sometimes fail to render your page on certain selections and can also take a long time to render but it is very helpful to see what it will look like on more obscure operating system and browser combinations.
Litmus (external link) is a website that offers paid for and free versions to show you how your web pages and emails render in various browsers and/or email readers. It is more reliable than Browsershots but the free version’s list of browsers is limited.