<p>I know that attributes are extremely useful. There are some predefined ones such as <code>[Browsable(false)]</code> which allows you to hide properties in the properties tab. Here is a good question explaining attributes: <a href="https://stackoverflow.com/questions/20346/c-what-are-attributes">What are attributes in .NET?</a></p>

<p><strong>What are the predefined attributes (and their namespace) you actually use in your projects?</strong> </p>

```
[Serializable]
```
is used all the time for serializing and deserializing objects to and from external data sources such as xml or from a remote server.
More about it here.