If I have one complaint about Linux, it would have to be the lack of powerful Computer Aided Design systems for the platform. Almost all of the common commercial packages such as AutoCAD, SolidWorks, etc. are designed to run on Windows. Luckily, this apparent paucity of systems isn't as bad as it seems!
There are a number of programs you can use for doing basic CAD, and most are free. For example, for doing some basic mesh-based modeling Blender is very powerful. Unfortunately, it's not a solid modeler and isn't aimed at CAD work. That said, it's a popular modeling option for RepRap users. For more serious dimensioning and 2D work, QCAD is a good option. The community edition is free and available in the Ubuntu repositories. I use it almost every day.
For anything more serious, like parametric modeling, interference checking, motion simulation, and stress testing it seems there are no good free programs. The FreeCAD project seems promising, but is severely lacking in the basic functions needed for a production-ready CAD system. Luckily, there are some proprietary programs available for Linux which provide that!
Medusa4 provides a very interesting licensing scheme: Use of the system is free, but exporting drawings for production or sale requires what is essentially a royalty fee. I'm somewhat impressed with what I've seen of their interface from screenshots, but I wouldn't say it's the most eye-catching I've seen. I was unable to get the program working on my AMD64 Ubuntu system, but I'm interested to hear about other people's experiences using it!
BricsCAD is another proprietary CAD system which offers what appears to be a very sleek interface and powerful rendering capabilities. I'm not sure about its ability to do interference testing or any physical simulation, but it doesn't look positive from the site's information. BricsCAD licenses will run about $380 for a basic version. I've downloaded the 30 day trial, and I'm looking forward to giving this system a more thorough treatment in the future.
Finally, arguably the most powerful and popular CAD system available on Linux is PTC's Pro/ENGINEER. PTC is discontinuing the Pro/ENGINEER brand, along with several others, in order to merge their CAD product base into the new Creo system. The idea behind Creo is to break CAD applications up into smaller, more task-specific apps that allow a multitude of users to have purpose-built programs for their needs. From what I've seen on their website and from the launch event webcast, this system looks absolutely incredible. Mind you, I can't find any prices for the basic package; but from what I've heard, they're steep. It seems that PTC manages distribution through their resellers, all of whom are able to negotiate prices with you for your specific needs. Given that, there could be quite a bit of variation between different installs. YMMV.
So, given that basic overview, what are your preferences? What are your experiences with using CAD on a Linux system?
After speaking to a PTC rep about adopting Pro/E or Creo for doing mechanical design at Kestrel Robotics, I have to say that I'm rather taken aback by their price. They wanted in the $5,000 range for a single seat (although much of that is a one-time "maintenance" cost, and so could be amortized for larger purchases). It seems that for the casual user or small business, Pro/E is simply out of reach.