Web application development languages
I agree with Eran and Robert, but I'll chime in here for one language that hasn't been mentioned yet and I think would be useful for you to consider as well.
If you want scalability and long-term applications, I would go for ''structured'' languages, as Robert put it. However, I am also of the opinion that when developing web applications, because of it being a highly competitive and rapidly changing environment, you need productivity, as Eran stated, and Amer contributed to with the ColdFusion suggestion.
That being said, I'd suggest you look into the OutSystems Agile Platform, at , namely the free Community Edition.
The Agile Platform has the advantages of having the speed of development of a rapid applications development language, along with the power and scalability of an Enterprise-grade platform.
The way this is possible is because of the fact that the applications generated by the Agile Platform obey to industry standards - they can be either .Net or Java stacks, running on top of IIS/Apache and using MSSQL or Oracle as its DB infrastructure. This also means that there's no technology lock-in - unlike other RADs - and there's no performance hit of running on top of a proprietary platform.
If you are going to test drive some languages, I'd follow Eran's advice - to find a language that you feel productive in - and I'd also include the OutSystems Agile Platform in the list.