Dragnet website wireframes kit v0.91 is a common library for Adobe Fireworks and contains over 25 objects that are useful for rapid prototyping of websites. It is completly free to download and use. The kit contains common web design elements such as scrollbars, buttons, menus, alerts etc. Most of them supports the 9-slice scaling feature that is build into Fireworks for a better resizing of the objects.
Yes, it´s true that Adobe Fireworks already is packed with such elements that looks really nice. But we developed the kit because we wanted something more rought, something that looked more like sketches, so the clients or our user test participants wouldn´t get distracted by the design when we instead wanted them to focus on the function behind the web page or application.
This kit is no longer being updated, but feel free to continue using it.
There are several good in depth articles about wireframing (or prototyping or mock ups) but the short answer is that wireframing is basically sketches of a web site (or software if you like) that is made with pen and paper or a computer. Instead of starting with designing or coding a pixel perfect web site you´ll use quick sketches to test out different layouts before you head over to the real design. It very handy to use when you do early user tests or are explaining to your client how you want to solve a specific interface problem.
Want to know more? How about this meaty article over at Smashing magazine.