HTML QuickForm at NYPHP
Last night I heard my friend David give an excellent talk on HTML_QuickForm, a PEAR module for creating HTML forms. HTML_QuickForm does a very nice job of handling all the messy business of forms processing: validating data, printing error messages while preserving valid data, indicating which fields are required, escaping HTML, etc. It comes with a default template, but you can both define you own layout or hook into other popular PHP templating systems, like Smarty.
Best of all, despite all this functionalty, HTML_QuickForm seems very lightweight and not overly cumbersome.