I need recommendations for a good (free) development notepad to work in on my new macbook, mainly for PHP, AJAX etc.
if you are familiar with vim, or got the time to learn a fairly complex text editor i would recommend checking out macvim
Vim is great text editor for writing web code. It has a steep learning curve, but works on almost all platforms.
TextWrangler is a free Mac text editor, which has PHP support built-in.
There is also Aquamacs and Carbon Emacs for an Emacs-flavoured Mac editing experience. Carbon Emacs is closer to the Unix Emacs experience whereas the Aquamacs people have put considerable effort into OS X integration.
Geany (geany.org) is my IDE of choice. It's lightweight, open-source, and I use it both for programming and note-taking. It supports PHP, C, Python, etc.
I know you've request it be free - but really, I can't stress TextMate enough. It's fantastic and it's also cheap (around $30). It is well worth the price tag.
If you insist on staying free - try Eclipse with PDT.
I'd also suggest trying out Smultron. It's pretty lightweight, but it's a very nice (and free) editor. And, of course, it has support for PHP, JavaScript, and all the major web programming languages built in.