My advice is to pick a language and go from there. Start with either PHP, Ruby or Python. Once you are comfortable with the inner workings of the language, then move forward with an agile development framework. Depending on which language you finally chose you will find open source frameworks for each:
Ruby: Ruby on Rail, Merb
PHP: CakePHP
Python: Django
I truly think that understanding the language itself will help a lot before jumping into the framework.
After reading through all the comments, I do feel Bookmarklet is the way to go. However my only concern with Bookmarklets is there's no way on updating an user's bookmarklet automatically, so if you need to change something, you pretty much have to email all your users to remove their current bookmarklets and setup the new one. correct?
It was a great story though... Please put it back if you are reading this, Chris