I thought people might find this useful. Please feel free to add to the list.

ThinkingRock

http://www.trgtd.com.au/

GTD-Free

http://gtd-free.sourceforge.net/

MonkeyGTD

http://monkeygtd.tiddlyspot.com/

GTD Tiddlywiki

http://nathanbowers.com/gtdtw/

GTD Tiddlywiki Plus

http://www.checkettsweb.com/tw/gtd_tiddlywiki.htm

d-cubed Tiddlywiki

http://www.dcubed.ca/Welcome_to_d-cubed.html

These are the ones I have found in my web travels. None of these are online solutions.

Andy