Coders using WordPress, this post is for you. Here are a few plugins that should make your WordPress hacking life faster, happier, and more productive. What a promise, heh? :)
Plugin: User Switching
John Blackbourn freshly released this handy plugin with which you can instantly swap between user accounts, without having to log out then log back in as another user. This plugin should come really handy when coding something that needs to play nice with user permissions and roles.

Get it: User Switching.
Plugin: No Login
Toot toot! Here is my own No Login plugin. Its purpose is simple: never see the login form again and directly load pages of the admin area as user "admin". Cookie expired, or need to visit the admin area with another browser to check your neat CSS/JS work? No time to waste on the login form! (plus, admit it, you never remember if your test account on your test site has password "123456" or "asdf")

Get it: No Login.
Note: as of writing, "No Login" and "User Switching" are mutually exclusive (or to be more accurate, No Login enforces authentification of "admin" and does not allow user switching. But I really like John's plugin concept. I'll update the plugin so they can cohabit peacefully.
Plugin: Theme Tester
WordPress' Automattic's Donncha's contribution to this list is a plugin that gives users with sufficient permissions (“switch_themes” capability) a theme browser that will change the current theme for them only, while regular visitors will see your site as usual. Pretty cool if you want to check how your content suits, or does not, a theme layout before going live.
Get it: Theme Tester
Plugin: Theme Test Drive
Vladimir Prelovac's theme runs a similar concept: test a theme on your live blog without your readers noticing anything funky and unexpected. You can also quickly load a theme by appending ?theme=xxx (?theme=Default for instance) to your blog URL.

Get it: Theme Test Drive
Any other cool stuff?
If you use or know about another cool bit that may make WordPress coders more productive and happier, be sure to leave a comment!