We've had a lot of success with an open source product called 'itextsharp' - it's not so hot at the HTML-to-PDF but is good for carefully piecing together a specific PDF document (such as a receipt). It's also free, which is always a bonus. See:
http://sourceforge.net/projects/itextsharp/
Another quick and lightweight solution is XMLPdf, written by a software company called Ibex from New Zealand no less. This tool has a model where you create an XML template file, with HTML-ish tags in the schema. You can also define tags in the XML which you associate with objects in your .NET code behind. Kind of clunky, but useful in particular circumstances. Not free, but cheap. Can find it by searching for XMLpdf on Google.
