I've just posted a free addon for designers/developers that need a totally customizable contact form:

Some of you may already be aware of that addon (it's been up on github for a long time), but I recently rewrote the whole thing to be WAY easier to configure (there's only 2 files you need to tweak now), added some handy features (automatic dashboard reports with CSV download, spam honeypot fields, notification email "reply-to" for the form submitter), and significantly cleaned up the code internally (which makes it easier to customize if you need to add your own form processing logic).

This thing started out as my answer to the ill-conceived "External Form" block that is included in the core system (which I always uninstall immediately when creating new C5 sites for a myriad of reasons), but has accumulated a lot of handy features along the way. So while it's configured out-of-the-box for contact forms, I also think it's a good starting point for all sorts of other custom forms (as long as you don't need to give users the ability to configure the form themselves).