UDG Plop is a small simple application for Windows, Mac OS X, FreeBSD and Linux that allows you to easily create User Defined Graphics (UDGs) for use in a Sinclair ZX Spectrum program.
The interface is easy enough: simply mark or unmark the pixels you want to design your UDG then, copy and POKEs or DATA from the right-hand panel into your Speccy program. You can also load and save your UDGs.
Download the Windows application here (800k), and the source code for building on other systems is here (15k).
Bugs and feature requests can be reported on the sourceforge project page or to me on twitter.