I got some very strange suggestions from people, from having a full blown art department dedicated to producing those to drawing the diagrams on physical paper and then taking pictures of that on a wooden table.
The truth is far more boring, I am afraid.
I generally use the following tools to produce the diagrams:
- Google Image Search
- Power Point
- MS Paint
- Visual Studio
I use visual studio's class diagram designer to create the class diagrams, usually from empty projects, not the real ones. Then I copy the image to power point, where I do some mixing & matching.
Power Point is really nice in this regard, because it offers a rich set of effects that even a graphical dummy like me can use effectively.
I use Google Image Search to find relevant images, and then drag them into Power Point as well, making the same style of effects there. Then, if needed, I move it to MS Paint for final processing, and from there, to the blog.
This approach works well for someone that needs three tries to draw a straight line using a ruler, it is also something that doesn't take very long, which is also important.