When a feature is added or an efficiency-improvement is implemented, the new code isn't always bug-free.  Sometimes nasty new bugs are written in, or previously bug-free features are broken somehow.  Quite often a problem will go unnoticed until a user reports it; so rather than only having one version up for download at any given time, I keep a "stable" version for people who just want as reliably-working a program as they can get and put the latest builds up for download as the "development release" for people who want to get at new features or new fixes as they become available.

Even though the program and its source code are freely available, I still hold the copyrights to it: it's distributed under a new-style BSD license (i.e. the three-clause revision), but legal mumbo-jumbo aside, the gist of the license is that you can use my code for anything you want provided you don't claim authorship, you don't attempt to profit from it without the author's explicit written consent, you don't use the code in any projects other than those whose source code is publically available, and long story short you don't in any other way violate intellectual copyright laws.

It's just plain polite to let somebody know if you find their code useful and include it in another project though, so while it's not necessary, I'd appreciate it if you dropped me a note here and let me know.