I used CombineZM (which is free) before moving to Zerene Stacker (which isn't). I read
here that Combine ZM was replaced by Combine ZP. It looks like no updates have been issued for several years, but that doesn't mean it won't work of course. It is a bit limited (won't handle 16-bit images for example, and doesn't have retouching facilities like Zerene has to let you fine tune the difficult bits, but worth a try I would have thought if you want to experiment with stacking.
Incidentally, you don't necessarily have to use lots of images for stacking to be useful. I don't use stacking a great deal, but when I do it is often with just two or three images, generally ones which haven't been captured with stacking in mind but just happen to be suitable. For example, this one used two images where the centre of focus happened to be a bit different.
(For a larger version click on the image and then right click and select
Original

0461 04 2011_10_02 P1210942,3 ZS-Edit-Edit by
gardenersassistant, on Flickr
And you don't have to capture the images in a closely controlled environment. All the stacks I have been done used images captured out "in the wild" with a living subject. On the other hand, if you want to get amazing images of a fly's eye or similar using 100+ images then you do need a precision approach with very fine control.