Work flow for georeferencing imagery using open source tools?

GDAL user-friendliness is basically nonexistent, but hot damn does it ever work well. I wrote up this guide to my own georeferencing experiments a few years ago: http://mike.teczno.com/notes/flea-market-mapping.html

It's a bit out of date, but the basic elements are there: find matching points between your image and a reference map (I'd now recommend http://getlatlon.com in favor the one at http://gorissen.info), use gdal_translate to knock up a virtual raster, and then you're basically done in the sense that the resulting VRT file can be converted to the GeoTIFF or tile of your choice.

I'm doing a lot with this right now, including collaborating with Tim Waters on the excellent server-side Map Warper mentioned in this thread, so there may be some new stuff in the near future loosely based on some experiments I did in JS last year: http://mike.teczno.com/notes/canvas-warp.html

I do know of two web-based solutions that might be worth taking a look at:

• Map Rectifier from MetaCarta
• MapWarper created by Tim Waters

I'm pretty sure Tim Waters open sourced his code, so even if these particular tools don't suit your needs, looking at their source might give you some insights.

Sorry, I can't post more than one external link because I am apparently less than reputable.

I find QGis's georeferencer to be pretty decent for a point and click tool. I wrote a little guide - image georeferencing with QGIS - which is slightly Canadian data-source specific, but walks through all the steps you need to get an arbitrary map into QGis.