This is where you can download drupal, get modules and so forth. I like drupal because
1. it's free
2. it's open source
3. once you learn its logic, you can design it exactly as you would like it to be
4. because they have a little monster drop as their icon. you may think that is not important - but it is a testament to the playful nature of the community. if you are developing for educational purposes, you may find the drupal community more welcoming.
5. i also think that people attracted to drupal are a little more 'bleeding edge' in terms of new types of user interaction support. for example, in version 4.6.5 i had downloaded the 'friendster module' (go join go) - which became incoporated in the most current version, and is fully modifiable.


mambo is another popular cms  - it seems to be neck and neck w/ drupal, but it probably wins out in more business-like situations. if i had endless time, i would test it as well. for now, i am happy with drupal.
of course, they all add similar features constantly...