Hi everyone,
For gimmick's sake; I'd like to create small apps whereby the user writes their name, for example, from an on screen keyboard, and from that produces a unique musical sequence.
How would I go about getting software, a website even, to communicate data with PD? Or any other software for that matter? That might well be an impossible question, sorry

@peeshder Hello Peter.....
You mention users on a website........ so maybe
http://funktion.fm/post/present-and-future-of-webpd will tell you what the possibilities might be........
There are a few people trying to push this forwards and so you might find something useful by googling around... the author of that link gives a fairly good summary......
the stuff (as it is now) is here.....https://github.com/sebpiq/WebPd

You can have Pd accept user entry and so do what you want within a Pd patch.......... but I get the impression that is not exactly what you are looking for.......
David.

maybe have it communicate with pd over a network with netsend & netrecieve

You might also want to look into OSC, if the other software you mention supports it. Pd does, and it's probably the easiest way for network communication.