For me there are two categories of "work in progress". The first (as Robert said) projects that never end. The best example:
Searcharoo.net
which had 7 iterations but still has a long way to go if it has to complete all the tasks it has to complete. I will continue to work on the code, as long as people continue to email me about this, but it goes on for months without any attention; then I could focus on it for a week before another long period. The main thing that makes me work on it is an interesting problem to solve . Mundane errors or "simple" functions or embellishment of the code never motivates me in the same way as doing something complicated.
Other things that I started because they seemed cool / funny, but stopped when I solved the initial problem and got stuck in the “common code” (you know: membership / login, reliable error handling, setting the user interface) include:
RaceReplay.net - which was originally written in WPF / E :) and updated for SL1 - although it has recently been revised, has been activated since the installation of MS SL VE Map Control. Sat for more than 12 months without any action, although
RecipeNow.net - originally written in .NET 1.0, updated for version 2.0 and is planned to be rewritten in MVC (one fine day ...). I even started (but not finished, of course) the iPhone version. There are grandiose plans for membership, contributions, sharing, community ... it all started, but it did not end ...
SilverlightEarth.com - MS / google viewer written in Javscript and WPF / E; SL1, long before DeepZoom. Support half ink. Half turn the card. Half-done SL2 migration ... half-working code is on the website so everyone can see (this is Javascript - just a source view to see all the crunchy hacks and TODOs)
Geoquery2008.com is the SQL 2008 geospatial query tool. Spent a whole bunch of time in about two months, getting 80% off the path to a “real” product ... but it has since stalled as an “almost useful” tool.
DeepZoomPublisher.com is very sad since I wrote the “Console Application” in the background (and posted examples of how it works), but I didn’t get to writing the promised Windows user interface and did not publish the code itself ... so if you go to the site, you will see examples of what it does, but you cannot find an application that does this :-( my bad.
To answer another part of the question, I never intentionally deleted them, although I “accidentally” tidied up my computers and somehow deleted all copies of one or two “semi-finished products”. VERY upset about myself - because I like being able to come back to finish them ... even if I don’t; -)
Network solutions are pleased that when I am delighted with a new project, I will register a domain name for it.
I think that blogs - especially those where the author is knowledgeable and amiable enough to write small examples / samples - replace "unfinished projects" for some people. If you have a cool idea, you just copy the minimum to demonstrate how cool it is - post it on your blog and leave it to it! (unfinished) without fault :-)