Is an IDE a must for web developer?

You can use what you want.

I prefer notepad (standard windows edition) or if I have to indent code, i use dreamweaver.

Its not a must
Sure, when you work on really big projects they can help, but you wont need it

I am using NotePad++, but I'm looking for a small IDE with some basic features

For little changes / programms, notepad++ is my choice. It has syntax coloring for every language i'm working with and has some other nice little features that make life easier.

For bigger Plugins im using Eclipse ( http://www.eclipse.org ), since it has everything a really good IDE needs and is free.

Well, that depends

Of course i'm using the "Eclipse IDE for Java Developers" version for the development of my java programms. For the development of wcf packages i'm using "Eclipse for PHP Developers", since wcf is written in php. everything else wouldn't make any sense

Quote from Alexander Ebert

it's just bloated and incredible slow (even on high-end machines) …

Only, if your machine is from the stone ages

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Do you know netbeans?

Back in the days, i have tested netbeans several times. but i never became a friend of it.

There were several alternatives to eclipse, of course. Zend Studio is one of them. But Zend Studio has a cost of at least 299,-, while eclipse is free. phpDesigner also is an alternative, but also not free.
Eclipse was originally develoed by IBM, and they still support the development of eclipse and use it for their development purposes. That said, it's quite a cool and powerfull software, with many plugins, so that you can easily customize it;)
But for me it works "as is" very well.

For CSS editing, notepadd++ is quite fine. But every solid IDE will support CSS editing Maybe not with all of its full features, but at last with syntax coloring.

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Not my quote but i would like to add another possibility which is netbook

Well, i wouldn't want to miss the comfort of my good old keyboard But even on a netbook, eclipse should perfom fine. It might be more ressouirce-hungry than other IDEs, but overall it is not that bad

Quote from Alexander Ebert

Wouldn't call a state-of-the-art computer (Core i7 + 12 GB RAM + SSD + Radeon HD 6870) a "machine from the stone ages" I just can't believe the overall slow performance, especially if one single action can freeze the whole interface. Why are all those developers such retarded, that you let a pretty "expensive" operation run in the applications main thread? At least Java provides a great API for handling threads, why just blow it?

Well, than you have a way better system than i have But i have not really encountered any troubles with eclipse, except for some slow performance in the first moments (using code completition) after adding many big new javadocs (were talking 15.000 + classes with documentation).

After all, i never understood those damn discussions about speed. as long as the user is not faster as the software, its fine (never understodd the discussion about performance between chrome / firefox though). and eclipse is fast enough (what difference do some spare parts of a second do - not a significant one for me).

But yes, there are many alternatives. phpDesigner is one of one of em. But not the only one. The good one about the free ones is: You can test em all and choose the one, that fit your needs best.

For quick testing I use Notepad++
For more serious work I use Dreamweaver. I also use Microsoft Visual Studio 2010 Express (the free version).
It all depends what I'm doing, the purpose why I'm doing it and how fast do I need it done.
I use all these tools to code in JavaScript, CSS, XML, HTML, C#(not dreamweaver), PHP, Ruby
The purpose for using Notepad++ is to learn and practice code, if I'm working on a project I prefer the DW or VS2010 because of their functionality.
I only use Notepad++ if whatever I'm doing is temporary and for learning or testing purposes.

I have phpDesigner It is really good also I use VS 2010 and 2008 for APS and C# programs.