Depends on the type of web application you were thinking of building. If one already has a system in place using Java, like many companies do, and you are creating an online presence for it, Java would probably be the way to go. If one is building a web hosting application and need some fast processing on the server side, I could see Perl coming in extremely handy, especially if you have an experienced server administrator giving you the tools to use it well and fast.
Obviously, EllisLab is very PHP focused and all our current software offerings are based off of it. It is very widespread, easy to learn, and there are plenty of useful resources out there for learning. Of course, the developers of the PHP language have been making a few missteps in the past few years, in my opinion, and it will be interesting to see what happens. On some level, I am kind of hoping for a fork of PHP to happen. Also, if Yahoo (much PHP in them) does get taken over by Microsoft, it could spell the end of their usage of PHP or it might mean that PHP will work even better on Windows server. I am hoping for the latter.
Python is a pretty darn swank language, and if it could gain more usage I think it could be a serious contender to PHP in the web application world as it picks up steam. Wish mod_python was more readily available on web hosts.
Ruby. Hm. I have never really spent much time with Ruby as so many people are using Rails to develop in Ruby and I never really liked using Rails, even though it does seem like it would speed up application development rather nicely.
You know, with many desktop applications moving onto the web, I keep on wondering if we are going to see a resurgence in lower level language like C. Also, you might be interested at looking at Adobe Air.
