Xref clipping has been around since AutoCAD R14 and is really useful. It has been improved upon last time in AutoCAD 2009 with ability to more easily edit the boundary as seen in this previous post that also shows the invert x-clip boundary that was introduced in AutoCAD 2008.
JTB World's XClipClean can help with the work to clean up the blocks or xrefs that has been clipped by automate the tedious work deleting and trimming geometry that is outside the xclip boundary. This can be useful for as build drawing production if you need to bind the xrefs and want to minimize the size of the drawings. Think of it as an clipped xref purger removing what anyway is not seen.
Did you know that you can clip a 3D model also in Z direction? Here is one solution.
I use the sample 3D House as an example here. I attach it to Drawing1.
Then I clip it from the top view like this.
In this step I have attached Drawing1 into a new drawing and change the view to a front view and clip it again.
And here is the result.
If you just want to clip the already clipped xref or blog parallel to the clipping boundary regardless of current UCS you can use the Clipdepth option that sets the front and back clipping planes on an xref or block. Easiest is if you are in a front, back, left or right view when you do this.
Imagine you work with a large project with multiple disciplines and maybe just want to use a small part of the building as underlay to keep good performance on the computer. This is just one practical usage.
Clipping a 2D xref also works with nested xrefs and can be really useful too.
If you work with clipped external references consider how to use INDEXCTL. Using INDEXCTL set to 3 can improve the performance up to 10 times I have seen, especially in combination with xrefs, xclip and viewports.
Feel free to share your tips in the comments or add a link to your blog post on the topic.