The biggest thing seems categories would be much
faster to scan if the categories were listed vertical
instead of all horizontal in a flow layout.  Notice in
particular "Developing NetBeans  Distributed
Applications" and "GUI  J2EE Tools", and the rest is
just a scanning issue.  Standard usability guidelines
could be applied and make it much easier to read.  

I just now, today, finally noticed you can click on
the cloud to change the organization of the layout of
the categories.  The graphic really needs to have some
sub-text to explain it better as it isn't a common or
well known icon for this type of action.  The layout
graphic within the graphic is hard to see on higher
resolutions.  There could even be some "smaller" than
the categories text links below "Categories" instead
of below the categories which could just spell out: 
Change category layout to vertical
Change category layout to horizontal

It should probably default to a vertically organized
category view and horizontal be the optional view. 
The switching stuff being below the "Categories" word
will make it easier to locate in both views as well.

Compare for instance this usability change I made for
nbextras:
http://wadechandler.no-ip.com/~wchan/nbextras20060618/
to
http://www.nbextras.org

I put the menu on the left playing around, and it
doesn't seem to matter so much if it is on the right
or left once it is noticed, but the biggest issue is
being able to quickly say this is 
1) this is one
2) this is two
3) this is three

without having to navigate with your eye through run
together.  Things are always easier for the designer
to understand, but they are easier for everyone when
standards for usability are strictly adhered to.  See
the vertical screen shot attached and just compare the
two side by side.