Pledges are how the hackspace gains a lot of its tools and equipment - the other major way being a donation.
When someone finds a piece of equipment that other members may also want to use, such as on our Tools Wishlist or if they feel the need to use something new and shiny (or old, we love ebay, and second hand tools in general), what tends to happen is that a pledge drive is put together.
Effectively, someone puts up a page such as Pledges/Heavy_Duty_Sewing_Machine, that explains the equipment, why this model is suitable for the hackspace (hard wearing, especially cheap, reasonably sized for the lift, fits in with the decor), puts a photo or link on the page to the equipment in question, and then maybe puts up a pledge of their own to start the ball rolling.
This pledge is then put out by the member on the [forum/mailing list] so other members can see it, and either pledge money via email or via editing the wiki.
Once the pledge target is reached, the item is bought, usually by the member who started the pledge drive, and the other members who pledged funds then pay that member.
This method of tool acquisition has allowed us to get some really significant tools, such as the Pledges/Frikkin_Laser, expensive inductive heated soldering stations, and more. Feel free to start a pledge drive!