Need a new writing tool: help?

I'm a no frills guy. I can write with just about anything and don't think the tool matters much. The tool is not the hard part. I write in Word, WordPress and now and then Notepad.

But at the moment I have 3 different writing projects going on at the same time – and I find it blocks me to have to sort out which project I'm writing for in a given moment before I start writing. And I write less.  I want to be able to write first, and sort it out later without copying and pasting or doing other medieval type things.

I want to be able to write in one tool, and just tag each post afterwards. Then a month later I can just filter on the right tag and get all the "posts" that fit that project.

Any suggestions?