Devel is a supremely useful module for Drupal development, but if you've never enabled the Development menu block, there are some useful links you might be missing out on. Here are some features of Devel that you might not know about:

Execute PHP
Path: devel/php
Provides a text area for entering PHP code into. Any output (print, print_r, var_dump) is shown in a drupal_set_message.

PHP Info
Path: devel/phpinfo
Get PHP configuration info from the server your site is running on.

View Theme Registry
Path: devel/theme/registry
Get down deep with the theme info your Drupal site knows about. Great for expert themers.

Function Reference
Path: devel/reference
Lists every function available to your Drupal site without loading any include files. Each function links to the API reference site you can specify at admin/settings/devel, so if it's a core function or a well-documented contrib funciton, it'll link to its documentation.

Reinstall Modules
Path: devel/reinstall
Uninstall and reinstall modules. This saves a lot of time compared to disabling the module, uninstalling it, and then re-enabling it.

Theme Developer Toggle
Path: devel/devel_themer
Another time saver. I love the Theme Developer module, but it's such a hassle to turn it on, use it for minute, and then off again right after using it. This path is a callback that switches the module on and off. Very handy.

View Source Code
Path: devel/source?file=sites/default/settings.php
View the actual, raw PHP source of any file on your website. If this sounds like a security issue to you, remember that every one of these Devel paths requires permission to view it.

Edit Stored Variables
Path: devel/variable
Edit variables stored in the {variable} table. These are the same variables available through variable_get() and variable_set().

Available Form Elements
Path: devel/elements
If you've worked with Form API, you've seen #type declarations like 'textfield', 'textarea', 'fieldset' and so on. This page lists all the types available to you. This can be handy with modules like gmap that silently add really cool form elements for you.

You can reference back to all of these paths at http://api.lullabot.com/devel_menu

2015-06-24 Update: You can now find the path reference for Drupal 7 here: https://api.drupal.org/api/devel/devel.module/function/devel_menu/7