I've been spending time with Jaxer, Aptana's JavaScript server.  I'm impressed.  I thought for a while that we'd see server side JavaScript eventually, especially given the popularity of Ajax, but it took until last month for a reasonable, implementation ready product to arrive.  And, thankfully, it works well.

Server side JavaScript is not new.  I used JavaScript...well...Microsoft's bastardized version called JScript, to build ASP pages in 2000 and 2001.  JScript's flexibility and extensibility made sense, but I never understood why it wasn't popular; why VBScript was the defacto standard for ASP.

After the demise of ASP, not willing to invest further in Microsoft technologies, PHP was really the only practical option, even with it's quirks and flaws.  But with Jaxer, the best of all worlds are available.  I can write all my scripting in a single language, use available, mature Ajax libraries, and reuse client side code on the server.  My first real project will be a gallery demo to see if I can replace my PHP galleries with Jaxer.