http://dummyimage.com/350×200

and viola!:

Any combination of height and width’ll work.  It’s simple and elegant.  And it gets better…

In addition to height and width, you can do all the standard ad sizes, screen resolution, even video format.  For a full-sized banner, just use:

http://dummyimage.com/fullbanner

Different colors? alt text?  jpg or png? just as easy:

http://dummyimage.com/400×100/0099ff/CCCCCC&text=TheNietz

Too lazy to save your own images and don’t want to link to their site?  They have that covered too, just download the source (MIT license) and run it on your own server.

If you do wireframes, this is a must-add to your toolchest.

[dummyimage] via [lifehacker]

Stellarium is an open-source planetarium on your desktop.  More than just a starchart, Stellarium can take a point in time and space and see the heavens.  Curious what the sky over Bethlehem was like in 32 BC? Just enter the coordinates, the date and time.  Not just stars, but the planets comets, nebulas, you can even incorporate weather conditions.

Stellarium can even output to a fish-eye projector for actual use in a planetarium (or you can buy your own pre-loaded with Stellarium).

When I traveled to Tibet last year I installed Stellarium on my Macbook.  The night before traveling to Base Camp, I was able to pull up my exact location and map out the stars over the Himalayas.  Something very cool.

[Stellarium]