You know how to deploy jars and wars on tomcact, you can hack .htaccess for apache and even speed up performance with nginx or lighttpd, but guess who doesn't know or care about any of this? Your customer. They care about their bottom line and whether you can do the work or not.
Customers just want you to fix their stuff, they need you to get in and fix or update or change things around. To them these kind of changes are always "quick" and "should be really easy for you". But before you commit to any hard numbers on your quote or proposal, be sure you understand exactly what technologies you're working with.
This is where wappalyzer comes in. Wappalyzer identifies software running on websites. It runs as a Firefox addon, and it does one thing and it does it well. Wappalyzer tells you all the different technologies running on a website. This is crucial for webmasters, webdesigners and even systems administrators that run their own businesses because it allows them to identify the type of work and skills that will be required to complete the job.
A really skilled systems administrator or webservice company might be able to obscure some of these technologies (and you should for security), but most people don't, and even if they knew how, they won't. So you can run this addon on against a website and find out what's going on before you commit to the work or come up with your estimate.
It's easy to install, just find Wappalyzer on the Firefox addon directory and install it, reboot and then Wappalyzer just works.
Take a look at these examples of websites and what technologies they're using. The red dots indicate the system being used.
The one thing that it doesn't do is find the hosting company. But there's another tool for that, that will be another topic soon.