CorelDRAW is arguably one of the most popular vector drawing programs available. It can produce a fantastic amount of graphic content, and while everybody is focused on a Photoshop replacement, vector drawing tools are often more important. They allow the production of scalable graphics that can be resized at will with no loss of quality.

Under Linux, Inkscape provides the same functionality. It doesn't replace 100% of what CorelDRAW can do, but it's amazingly good. In fact, it's one of the best-written Open Source products I've seen. Fast, efficient, and does its job quite well.

Adobe Photoshop is a hot item on everybody's to-do list. The truth is there's already an excellent graphics editor called The Gimp, and it's been available for some time. The only thing it lacks is some of the sophisticated filters that come with Photoshop. However, these are primarily used by true artists. For the basic image cropping, retouching, and conversion that most of us need, The Gimp does an admirable job.

My only complaint is that it's not very intuitive to use. Come on, guys, is this THE most non-standard GUI that you could possibly make? Some functions you can get to via the menus, as you'd expect. Others you have to get to by right-clicking the image. It's not always completely clear which is which. If you want a simpler tool, more akin to MS Paint, just for cropping and conversion, look at Kolourpaint. It's not very sophisticated, but it does offer all the basic drawing tools in one package, and it's actually much easier to do learn tasks like setting image transparency.

AutoCAD is arguably the most popular CAD program out there, and it does a lot more than just some 2D line drawing. However, it's also an $800 product for an ENTRY LEVEL option, and it's somewhat unfair to compare free options to something of this magnitude.

If you need a really sophisticated CAD application, there are commercial options that run on Linux. But, if you just need to draw some lines and other primitives, dimension them, and use that to draw up your next MAME cabinet, take a look at QCad. This is actually a commercial product, but a free version is available for Linux (and Windows/Mac OSX). It does a relatively decent job if you just need some core functionality. You get a lot more than you pay for.

There are many commercial 3D modeling applications, and Truespace is just one example. But in this category, don't think "alternative", because Blender is right there with the best of them. There's really nothing you can't model with it, just be prepared for its somewhat unconventional GUI. I'm not sure why some developers insist on trying out weird GUI experiments - in Blender it's the tool bars - but to each his own, I guess. Again, you get MUCH more than you pay for here.