Amongst the software that I use are two little gems from HP.
HP Product Bulletin
This is basically a full guide to the HP Product line, both present and past. It contains all kinds of useful information including tech specs, memory option part numbers and the like. Really handy for when you need to check certain configurations are valid and the like. You can download it from here http://h18004.www1.hp.com/products/quickspecs/productBulletin.html
The second cool tool that I use is the HP ConfigureAider. This tool is for current systems only and is a wizard based tool that you can use to configure HP Servers from start to finish. As you walk through the wizard, it will allow you to adjust the configuration of your server and finishes with a complete parts list and RRP for your server. It makes life a whole lot easier when your constantly building various configurations of servers all the time. The ConfigureAider is available in both an online and offline version. You can download it from here http://h20427.www2.hp.com/products/configureaider/au/en/