Since the birth of Web 2.0, a lot of technologies have been stepping up into the lime light. Some of which are CSS, Javascript and XML (more known as AJAX).
But for those of you who still remain in the dark and do not know much about this, let me start by giving you hints. For developers who love PHP, you might want to try using Symfony for development. This will open your doors to MVC and Rails programming and introduce you to AJAX too. But, if you are more adventurous and love more spice.. then why not try Ruby on Rails? 
I made mention these two because PHP and Rails seem to be the most common programming scripts used by developers. I myself use them.
CSS too is becoming popular with various Social Networking Sites, offering their subscribers with self customizable sites. Even the ordinary people who wanted their sites designed are now very much into CSS. It has eliminated the heavy use of tables for designing pages, and has given flexibility, style and fluidity with designing. With the use of CSS, even designers and programmers can separate their tasks ergo reducing much of development time and speeding up delivery of quality outputs.
AJAX is cool too, but to developers out there who are still so much fascinated with AJAX, please be reminded that you must also support javascript disabled browsing. This is the main pitfall of sites powered by AJAX applications. This may actually not be necessary, but also helpful at times. Its tantamount to still having the snail mail vs the email. There are benefits of using the old school style of doing things vs the modern ones. Some maybe for security reasons, while others are just for the art of practising discipline. But whatever it is, don't overdo things. Remember always that there must be two sides of a story.
I've seen sites that work seamlessly when Javascripts are on, but there are times when I do turn off my Javascript for personal reasons. I hate it when the sites suddenly don't work. Gmail may be mentioned as a good example that implements AJAX the right way. It offers its users two versions: The standard version is the one that is 'feature-rich' and uses AJAX, the other one is a plain html version. Nonetheless, the latter works just fine. So, all developers out there, I challenge you to be considerate enough for your users. If you offer coffee, make sure you also have tea. Makes sense right?
Though CSS and AJAX prove to be much savoured technologies to date, remember still to keep your feet on the ground. Not everything must be done via these technologies to achieve the purpose of deploying your site. Each site has a different goal to achieve… so, one man's treasure can't be everybody's gold.