If you ever tried a window manager that doesn't use overlapping windows, you will have found that some programs create too many windows to work well with it. You can let these programs use an X server that displays in a window of your first X server.
Note: this trick is useless if you want to make a vector drawing. If you're using inkscape for something that should eventually become a .png or .jpg or .gif or etc etc, then it might be useful.
When drawing really complex shapes (such as long texts on paths), inkscape can become very slow and may even crash. You can work around this by isolating the complex shape into a separate drawing. Make sure that drawing has a transparent background (file - document properties - page - backround should show a block pattern on the left and a solid color (usually white) on the right)). Export it at the same resolution as you want to export your final drawing (or maybe the double resolution is better, especially if you want to export at a lower but higher than half resolution too). Delete the complex shape from your original drawing, and import the .png you just exported to replace it.
Every once in a while there comes a situation where a file is too big for a CD, or you want to store a file larger than 2GB on a FAT partition, or you want to make a backup of data with UNIX-style permissions (almost a 100% necessity if backing up a $HOME folder with settings) and all you have is a FAT volume somewhere. Here the multi-volume feature of GNU tar comes to play. The ZIP and RAR formats have a similar feature, probably originally for floppy disks, but they don't store UNIX-style permissions. Also, the feature is in GNU tar for a partly historical reason (partly because it is probably still widely used for the original purpose):
If you find text on a website too small to read in FireFox you can increase the font size by using Ctrl and '+' on the numpad. This will only effect the tab you are currently in, and you can revert with Ctrl and '-'.