Current Options for Atari 2600 User-programmable Cartridges

Here is a list of options for user-programmable Atari 2600 cartridges. 

Harmony Cart: http://harmony.atariage.com/
Melody Cart: http://www.atariage.com/forums/blog/107/entry-6412-perfect-harmony/
DIY EPROM / EEPROM Cart: http://www.atariage.com/2600/faq/index.html?SystemID=2600#pinouts
AtariAge / PixelsPast EPROM Cart PCBs: http://www.atariage.com/features/shows/atariage_pc5/pixelspast.html
Krokodile Cart: http://www.atariage.com/software_page.html?SoftwareLabelID=2716
Cuttle Cart: http://www.schells.com/cuttlecart.shtml
Modified SuperCharger: http://www.biglist.com/lists/stella/archives/200203/msg00022.html
Modified VecFlash Cart: http://little-scale.blogspot.com.au/2006/08/at-last-loopcart-on-real-atari-2600.html