Does anyone know any simple and effective eCommerce scripts? I don't need anything fancy, just the usual order processing and item galleries, and preferably with easy layout customization.

I recommend the oscommerce software very easy to use and do customization.

Do you already have a website running? If not and you want a CMS try out ubercart for Drupal from http://www.ubercart.org

I have set up a free apparel online store in fastcommerce.com two days ago. It is used very simply. It may be helpful for you.

Hi,
You can go for OScommerce , E-Php (CMS) or Shop-Script PREMIUM.OScommerce is world renewed and easy to manage your products and daily routine tasks.But I will also recommends you to check Shop-Script PREMIUM because I also found this CMS good because it is same like OScommerce with some extra futures. i.e Client notifications though SMS is best future I found while working on my client project.

OScommerce as others have mentioned should fit your needs.
Mike

There are a few shopping carts that I recommend you to take a look at, namely Magento Commerce, OsCommerce, ZenCart and Cube Cart.

I would probably recommend OsCommerce also for your needs- but make sure your hosting company will support it. And not sure "say" they will support it, but actually help you get it running. It can be painful.
If you ever want to switch to an on-demand eComerce solution there are several providers and I can help shed some light on some :)

I've heard good things about Magento.

I used to think Oscommerce is not enough good but now i realised how powerfull system.After using oscommerce my mind changed.If you make some modifications oscommerce will be wonderfull.
I recommend OsCommerce.I've a website running oscommerce.
<<snipped>>

My vote would go with Magento (http://www.magentocommerce.com/) as well.
You can look over their live demo at the URL below. It looks quite nice and has been featured in a multitude of places.
http://www.magentocommerce.com/demo

My vote goes for ZenCart :P

OsCommerce all the way to the bank :)
By far best free open source stand alone shopping cart script. Easy to install and configure, open source, tons and tons of developers that can help assist you. All the major ipn integrations and not to mention support for all the major shipping companies.