Heeks CAD is an easy to use and quite functional CAD software:
http://code.google.com/p/heekscad/
Its a bit too much focused on CNC I think, but not so much that it limited you for other things. And even though it's learning courve is slightly higher that Sketchup (but not much) it make for a nice 3D drafting application also.
Other software I like and which is really useful (or at least cool):
Quantum GIS: http://qgis.org/
which is a quite fully featured geographical information system (GIS) software
PiTiVi: http://www.pitivi.org/
Which is a nice video editor
Zentyal (former ebox): http://www.zentyal.com/en
Which is a really nice linux based home/office server system
MintLinux: http://www.linuxmint.com/
My favorite operating system
Mixx: http://www.mixxx.org/
Awesome DJ software
EPANet: http://www.epa.gov/NRMRL/wswrd/dw/epanet.html
Software to design water pipeline networks
ImageJ: http://rsb.info.nih.gov/ij/
Scientific image analyzing tool
GENtle: http://gentle.magnusmanske.de/
DNA editing tool
Freifunk: http://start.freifunk.net/
Firmware for WLAN routers to set up a mesh network
Add more cool things if you like
