Working on a project right now and I have a few must have controls that I include in most sites I build. Often times it strikes me that the best controls are not always that well publicized so I thought I would ask the SO community about their favorite 3rd party asp.net controls - the ones you just have to have on a project because they are so valuable.
EDIT:
Some people have asked for clarification and the controls that I include
First, I am not talking about just UI controls but libraries as well, a few people have mentioned log4net which falls into this category.
For me, there is one 3rd party control package which I find to be masterfully done and an incredible value for the money - it is Peter Blum's Data Entry Suite. As Scott Hanselman says, it is a "complete re-imagining of the ASP.NET Validation framework" and it is loaded with controls for rich interaction with data entry forms.
Outside of that, I am really digging JQuery lately but like most of the answers so far also use the ASP.NET Ajax Control Toolkit in many places.