My expertise covers...
Most of the server-side web application development today happens in PHP, and I do lots of that.
Then there is the Bayeux protocol, that works over ajax/comet and allows real-time multi-point communication between clients. Combine it with an advanced framework like SproutCore for a world-changing mixture!
For years I was programming in Java when we were developing Austria's first real-time trading system. Recently I'm developing interfaces in it for connecting Java-based systems to the rest of the Internet.
Concurrent programming is not easy. Distributed programming is even harder. But Erlang, the language and the platform, is a very potent foundation for some interesting software, like -- for example -- mixing Erlang and "share nothing" scripting languages like PHP to create best of both worlds applications.
I love Erlang's ability to trace the workings of live systems, the ability to upgrade programs on-the-fly (!), the clean code that results from non-defensive programming (because error handling is in the supervisors), the Mnesia distributed database, the code loading over network...
I used to program a lot in C/C++, and it's still one of my favorite languages.
I did systems programming like device drivers for video capture hardware, an object-oriented Windows framework, and interfaces for wearable computers; and I worked on applications like capacity planning software for assembly lines, medical systems, and a CAD program.
Obviously, every unix guy uses shell scripting: bash, ksh, tcsh, and my favorite zsh.
TCL is one of the best scripting languages, but it just didn't hit critical mass. This didn't stop me from developing a Linux-based pay-as-you-go Internet terminal system for hospitals in it.
Python is another language that is getting more and more popular, especially with the emergence of Django and other python-based web frameworks. Interestingly, many of the Comet servers are written in Python, like Tornado, Diesel and Twisted. (Just like Erlang is used lately in such advanced software like CouchDB and Scalaris.)
Then there is PERL, that many call a "write-only language", since PERL programs sometimes tend to look like line noise. Still, it's legacy at many places, and it's some people's choice of a scripting language.
Communicating computers is a huge interest of mine. I know the Internet's protocols starting from ethernet and PPP, thru ARP, IP, TCP, UDP, ICMP up to the application protocols like DNS, HTTP, SMTP, IMAP and FTP.
I remember old protocols like xmodem, zmodem, kermit and bluewave; and I understand the newest ones like Comet, Bayeux and SPDY pretty well, because I know the foundations they are built on.