What is the purpose of the tiny ruler that I've just pulled out of the side of my Dell Latitude E6430?

It is a cool little multi-tool that Dell used instead of a plain slot keeper to keep dust and crud out of the Express card slot. It is actually pretty cool that Dell put this handy little tool in place of an otherwise useless hunk of plastic. There is a letter opener and everything.

Pretty cool.

Asus has one that stored flash Cards as a storage area:

And there is one that you can get if you are bored with your multi tool:

Seems there are a few other remotes you can store in this space.

A clever soul at Dell has turned the plastic spacer filling the ExpressCard slot into a small ruler, wire diameter measurement tool, and letter opener. This is maybe not an essential feature, given that the ruler is only 2in long, but it shows an amusing level of design imagination.

Read more: http://www.itproportal.com/reviews/laptops-pcs/dell-latitude-e5530-advanced-review

The Express Card (EC) slot is a PCI Express card slot used for various hardware options, such as a wireless card. PCI Express Card technology is the latest PCMCIA standard. There aren't very many cards (relative to USB) to put in them but you can add USB ports, media card readers, bluetooth and wireless cards.

Phone compnanies often hand them out as part of their mobile broadband offerings.

Solid State Drives (SSD's) are available, as well as Serial ATA (SATA) cards for adding external SATA drives.