Is there any good open source AutoCAD alternative that is used by (semi)-professionals or is AutoCAD the standard?
Depending on your requirements you might consider Google Sketchup, apparently some people have had some success running it using Wine. http://wiki.winehq.org/GoogleSketchup
It's been some years since I wrote software for AutoCAD but I was always under the impression that it was the market standard, with the possible exception of Solid Works which also has a market share.
If you are having to send drawings to other people or receive their drawings you will most likely find that they are going to be using DWG (AutoCAD) files and you will need a facility to view if not edit them.
The Open Design Alliance have tools for viewing and to an extent editing DWG files but I'm not sure about their level of Linux support.