In the process of writting the markup of a web document, you can inadvertedly make mistakes that will prevent your document from being conformant and that could even cause rendering problems. In such situations you can run automatic checks on your documents that will take care of finding possible errors, allowing you to repair them. For this purpose, I recommend the W3C markup validation service.
Keeping track of visitors accessing your website could be a great idea. With analytics services you can be informed about how, when and from where users access you website, what type of content were they looking for when they arrived, what page of your website they've landed in, and what page made them leave. All this information can be crital in any decision making process. In this section I'll recommend Google Analytics, because it's free and excellent.
A website can shine with a small amount of styling and a couple of great fonts. Up until recently, authors depended on the fonts intalled on users' computers to style their documents, and if they wanted to produce documents that could render consistenly in most configurations, the number of fonts available reduced to a small handufl. And this not considering the fact that many fonts in that small selection were very alike.
But with the evolution of CSS, web design has become more author-friendly. By today's standards, you can upload a font file to your server and, not without trouble, link to it from your document. Thankfully, some online services take away all the hassle and make available an incredible amount of fonts, free of charge and ready to be used. This is the case of my next recommendation: Goolge Fonts.
For many websites, a good set of high quality images could be what makes it stand out. But quality images usually come with a price and using free images sites, which are full of regular user-taken photos, may not be such a good idea.
Thankfully, this isn't the case with Pixabay, a free high quality images database. In this website you'll mostly find high quality images at absolutely no cost: all pictures in this website are released under the Creative Commons CC0 license as public domain.