I found Notepad++ to be a very simple IDE, which does the basics very well. But I did find that I missed a project/file list.
I found Eclipse to be feature packed, but very bloated.
Zend Studio is again feature packed, but had poor performance when accessing files over a Samba share.
Komodo had nearly all of the features I wanted in an IDE, including the project/file list. But I moved from that to the IDE I am using now...
Netbeans. I find this has a great balance between the features I will use regularly and those that I will not. It also has method and variable lookup functionality, which is very helpful when in large OO applications, with duplicate function names used throughout.
I've recently just transitioned to using Mac OS X from Windows, and was pleased to find it available on this platform too, but most of the IDEs listed above are available cross platform too.
Also if your on linux (I like ubuntu) then there is gedit. It is in many ways like the linux version of Notepad++ and usually comes pre-installed. Also if you want an equivalent for a Mac, then dual boot your mac with windows and you can get the benefits of the dark side of the force. Or alternatively you could use the mac as target practice at the fire range and buy a windows computer. But however you go about it perhaps you should use Notepad++. The ultimate text editor.