The size of the repo is a big disincentive to anyone wanting to participate in processing development, and this has got to be another example, a bug has been filed about the unecessary size of the repo  https://github.com/processing/processing/issues/1898 but I reckon you've got case to file another.

Same here evey time i tried but it works fine with

monkstone, it would definitely be better if the repo was smaller. Wasn't it smaller with googlecode? First I will wait for the reply of GitHub to see what they have to say about cloning big repo's.

Makio135, thanks for the suggestion. Indeed that worked in the sense that I did manage to download the complete repo. While solving one problem, it apparantly leads to another (see  below). Would love to hear it if you have a suggestion for that too.

The download turned out to be 1.4GB. But while it was downloaded, it is not working correctly under GitHub for Windows. It didn't show up in the local repo's and the online repo list still showed the clone option. I solved the first by dragging the folder into the repo list. Now it's visible, I can prepare a commit (which I did, fixed an issue, building with ant works fine). However when I press sync I believe it starts to download the whole repo again. To my unpleasant surprise after a while I noticed over 10 processes which had all already downloaded hundreds of MB's each (!) and were secretly draining my bandwidth. When I closed the GitHub application they kept going. It's pretty ironic that a code related service like GitHub has such a terrible application themselves. For now I submitted the solution as a comment with the issue instead of a commit/pull request.

I haven't tried the GitHub application (or only briefly, I didn't like the look and functionality, too simplified. And probably too tied to GitHub only).
I recommend the GitExtensions application, excellent: lot of features, yet easy to use.
There is also the BitBucket application (forgot its name), close in interface and functionality (but perhaps a bit less) to GitExtensions. I prefer the latter, but one advantage it has is that is also supports Mercurial.
One of my first attempts at Git UIs was SmartGit, quite good, but it failed to connect to GitHub with PuTTY (perhaps my fault, perhaps it has been fixed since then) so I dropped it.

Thanks PhiLho. I installed GitExtensions. It looks good and I've been able to easily open the local repo's, see the commits, add commits, etc. The problem I am having now is pushing a commit to the remote repository on GitHub. I used PuTTY to generate new keys and registered those with GitHub (later found my existing GitHub keys in %HOME%/.ssh). However I can't manage to have a solid combination of url, keys and other settings to make it work. I tried to use the following urls in Manage remote repositories:

URL: https://github.com/UserName/RepoName.git (this was the default setting in each of the local repo's)
RESULT: Unable to open connection: Host does not exist

URL: git@github.com:UserName/RepoName.git
RESULT: Hi UserName! You've succesfully authenticated, but GitHub does not provide shell access.

So how can I set this up correctly?

Use the second form. The " GitHub does not provide shell access" message just tells you can't do git commands directly on a kind of GitHub shell, which is OK. But the " You've succesfully authenticated" message means your keys has been recognized and you can pull / merge from the Processing repository, and make push requests too. With the GitHub plugin shipped with GitExtensions, it is dead easy.

Ok thanks, everything worked out in the end. After a successful test on one of my own repo's, I also managed to push to my Processing fork and put together a pull request with the fixes.