The all-powerful Jetpack plugin for WordPress is full of useful features, one of which is contact forms. It's a pretty good choice for this because it ties in with Akismet for spam protection, stores all submissions in the database (so your email account isn't the only place to look if one goes missing), and comes with a nice UI for creating the forms. Once installed, you get an "Add Contact Form" button on your Posts and Pages edit screens, with sensible defaults. Overall, it works well.

One thing you may not know is that you can put those contact forms in a widget. There's no built-in widget that does this for you (not yet anyway), but using a Text widget, you can use the same contact form shortcode to put a form in your sidebar, or anywhere else you can place a widget. Here's a video demonstrating how it works: