Responsinator
http://www.responsinator.com/
Multiple devices all on screen at once in a single long column page. Faded device images frame the iframes.

Responsivator
http://dfcb.github.io/Responsivator/
Multiple devices all on screen at once in a single horzontal page. Plain iframes. Nice customisation options. View random. Saves website in URL.

Responsive.is (by Typecast)
http://responsive.is/typecast.com
Single central iframe. 5 device size options. Saves website in URL.

Responsive Web Design Testing Tool
http://mattkersley.com/responsive/
Multiple devices all on screen at once in a single horzontal page. No device images just plain iframes. Either fit to screen height or show actual device sizes.

Responsivepx
http://responsivepx.com/
Single central iframe. Resizable via width and height sliders. Toggle scrollbars. Fluid resizing (and typed numeric). Saves website in URL.

Screenqueri.es
http://screenqueri.es/
Single left-align iframe. Resizable (horizontally & vertically). Lots of preset devices. Saves website as URL (but not explicitly). It has ads & is trying to sell me a theme.

Aptus
https://itunes.apple.com/gb/app/aptus/id510487565?mt=12
App: Dedicated browser that lets you preview, edit and screenshot.

Responsive Design Bookmarklet
http://responsive.victorcoulon.fr/
Bookmarklet: Single central iframe. 4 device options. Optional keyboard overlay.

Izilla Media Query Debugger
https://github.com/izilla/Izilla-Media-Query-Debugger
Adds the viewport width and height as a :before pseudo element on the body to help with obtaining values for responsive breakpoints.

Bricss
http://www.benjaminkeen.com/open-source-projects/smaller-projects/responsive-design-bookmarklet/
Multiple devices all on screen at once in a single horzontal page. No device images just plain iframes.

Screenfly
https://quirktools.com/screenfly/
Single left-align iframe. Resizable (horizontally & vertically via typed nuermical). Lots of preset devices. Rotate. Share.

Responsive Roulette
http://www.jordanm.co.uk/lab/responsiveroulette
Single central iframe. Resizable randomly by clicking a button.

Fontier for Chrome
https://chrome.google.com/webstore/detail/fontier/dkbamaalakfhckcidgiigdinhcncaeae
Chrome extension: A way to change the default font size in Chrome. For testing Responsive websites using em and percentage units. 

Resolution Test
https://chrome.google.com/webstore/detail/resolution-test/idhfcdbheobinplaamokffboaccidbal
Chrome extension: An extension for developers to test web pages in different screen resolutions, with an option to define your own resolutions.

Firefox Developer Tools (shortcut CTRL + SHIFT + M)
https://developer.mozilla.org/en-US/docs/Tools/Responsive_Design_View
Firefox dev tool: Single iframe (a-like) fluid resizing. Device preset dropdown. Rotate.

Viewport Resizer 
http://lab.maltewassermann.com/viewport-resizer/
Bookmark: Single central iframe. Fluid drag resize. Lots of device presets. Custom sizes (typed numerical). 

jResize
http://toddmotto.com/labs/responsive/
Single central iframe. Width presets. Custom widths.

Resize My Browser
http://resizemybrowser.com/
Creates popups of a certain size when you click links.

Juicer
http://juicecreative.co.uk/juicer/
Single iframe housed in photo-realistic images of devices. iPhone, Android, iPad only.

RWD
https://chrome.google.com/webstore/detail/rwddy/gkhnhkakdgogajfjmenpdeakhofjadhe
Chrome extension: Pops a little div at the top of any page that shows what the site looks like at 320, 640, 980 and 1280px wide.

ish.
http://bradfrostweb.com/demo/ish/
Single central iframe. 5 width presets (only they aren't exact!!!!). Custom exact sizes (via typed numerical). Website cached in URL for sharing.

Responsive Tools
http://responsivetools.com/
Multiple devices all on screen at once in a single long column page. Faded device images around the iframes. Device "jump to" link on left.

Am I Responsive
http://ami.responsivedesign.is/
Shows website at different sizes in an eminently screengrabbable way!

I Am Mobile
http://www.iammobile.co.uk/
Gives you a score out of 8 (?) for whether you're mobile or not. It's actually a really good check of a number of little mobile-friendly things you can do (as well as showing you your site in mutiple devices)

Responsive Inspector
http://outof.me/responsive-inspector/
Chrome extension. Digs into the code a bit more. Resizes your entire browser though (boo!)