This can serve as a general review of Tweakbench products, as well as a review, specifically, of Rebar, which for reasons I can't quite explain is my favorite of the bunch. Tweakbench plugins--effects and synths--are diverse, inventive, simple to use, great-sounding, and free. They all have a similar GUI, one that I absolutely love--clear, distinctive, and uncluttered; and Tweakbench is a one-man operation with great customer service.
Rebar is basically a metallic-sounding drone synth. That's it, really. But there are three things that make it really distinctive--the ability to draw your own waves (a new feature in the last couple versions), a "warp" panel, and a "delay" panel. The warp panel is a two-axis control that affects (I think) pitch and perhaps some kind of filter...the delay axes are for delay time and feedback. You can assign the cc's for these panels to a control surface, or the beam thingy on the Alesis controllers, or sliders, or whatever...the results are creepy, squiggly little grinding sounds...really cool and distinctive.
Tweakbench synths are one-trick ponies, and as a result I end up using them more than any other VST's I have. They do specific things in a characteristic way, and never fail to do those things. Most TB sounds could probably be reproduced on a full-featured synth or effects unit, but not with the same immediacy and fun--the simplest of them are like the Casios of the VST world. Others--especially some of the effects--really are unique and unreproduceable elsewhere.
Anyway, Rebar is awesome--there's no conceivable reason for you not to download it immediately.