Quickload (bundled with Quicktarget)
quickload quicktarget software
If you experiment with nonstandard loads there is nothing better. It does an excellent job of predicting the performance of a large selection of powders, cartridges, and bullets. It also predicts the effects of barrel lengths, seating depths, and powder temperatures which no loadbooks I've seen do.
Quickload does not eliminate the need to work up loads using a chronograph and shooting groups to find loads with best accuracy and low velocity SDs, but it certainly makes finding powder and bullet combinations which are likely to give good performance quicker and easier. Updates with new powders and bullet are available every couple of years with inexpensive updates for existing users. Bullet information is from manufacturers. Powder parameters are primarily from military lab tests. (German)
It runs on XP through Windows 7.
I rate it the "best buy" of all the handloading tools and accessories I use.