There are a good few options more than Foundation and Foundry these days.
Haven’t used Foundry, looked at it in the past and is didn’t quite suit me at the time, many happy users though.
Foundation is due to be replaced by Foundation 6 soon (you haven’t missed 5 versions it’s just a naming thing). If I remember correctly any purchases of Foundation now will get a free upgrade to Foundation 6, though the upgrade may not be straight forward and may involve rebuilding with the new version stacks (someone who is involved in the pre-release testing may be able to help with that one). The developer behind it, Joe Workman, is very good as he posts lots of videos and is very active in the community. I have the original version, it’s been good to work with and learn more about responsive design.
Source is one of the newest, I have it and I’m really enjoying learning to use it. The free stacks are a good starting point, the extra stacks which do a few more things and allow more custom layouts are fairly inexpensive (£20). I would highly recommend buying one of the training packs (the Vista one was very useful). It doesn’t do everything, that was never it’s intent, it’s more concerned with being as small and fast as possible whilst allowing you to add your favourite stacks as needed.
Platform is another recent addition. I have it but haven’t played with it much yet (never have the time I hope to have).
UIKit recently upgraded to version 3 and this seems to have been a very popular upgrade, particularly with power users. Starter projects can also be bought directly from Weaver’s Kingdom and also UIKitter as well.