Go to Setup tab, Company Setup, and check that 'Search Customer/Supplier/Item list' are unmarked.
If these lists are marked, then you have enabled search for Customers/Suppliers/Items.
This might be a good idea if you have many records. You can then start a search by pressing a space 
in the list, or use the edit box in front of the list if such one is available. Now enter fragment of 
Customer/Supplier name or Item id, category or description and press tab. The list will now be 
populated with relevant records. If you enter a *, all records will be shown.

In short, you can check the "Search Item List" in the company setting and then you will be in better position to select the right item when having "tons" of it.

That being said, a drop down box with categories would also be a great addition.

Gaston

The problem is that even if i use a lot of item categorys, i get a huge dropdown list when i want to write an offer. And to search for the parts takes too much time.

I would appreciate if there was a dropdown which let me choose the item category first and then only the products which are within this category.

Is there any solution for this?