As someone who has used PICKit 1, PICkit 2, ICD2, Promate II and Promate III and a follower of PICkit 2, I find it interesting that people still want to create yet another simple USB programmer.
Anyway, here are my classification of USB PIC programmers. I might miss many programmers in the lists but it pretty much show you the pictures. After class 3, then there are many high end programmers from Data I/O, BP or similar for mass production.
Class 1: Simple USB programmers (not as good as PICkit 2 and USBprog):
1. picsquirt http://www.p10link.net/plugwash/picsquirt/
2. GTP USB lite (somewhat like a USB version of Wouter's WISP628A) http://www.hobbypic.com/index.php?option=com_content&task=view&id=12&Itemid=35
Class 2: Good for personal and lab use
1. PICKit 2, good and cheap, supports many PICs and double as a debugger.
2. Another good USB programmer is the Amadeus.
The author is aparently very knowledgeable about PIC programming specifications. Not many people understand it as thoughly as him. Olin may be one of them. http://home.arcor.de/bernhard.michelis/
3. The author of WinPIC800 software has the GTP USB+ Programmer which seems to be a decent programmer as well.
4. USBprog is a kind of higher-end PICkit 2 in terms of hardware functionality. USBprog hits a good spot between PICkit 2 and Promate III but I find its chip support a bit limitted compared to PICKit 2.
Class 3: low quantity production quality programmer After class 2 then you have higher-end USB programmers like Promate III and SoftLog ICP2. SoftLog ICP2 seems to be a serious contender to PM3 at much lower cost.
Between USBProg and Promate III, there are still quite some good gaps. SoftLog ICP2 is a good attempt.