Tools & Technique: Paper Crimping
I use the Uchida Paper Crimper (link below) and I really like it. It will crimp up to 8.5inches wide (most crimpers are smaller) so I can send an entire letter size sheet through. Honestly though, I haven't needed that much width yet. And a paper crimper is not usually a tool you use everyday (but it's so cool to have!). So...I leave it to you to decide which brand/size you want to purchase. Both the crimpers I listed are known to be good products.
Here are some links to get you started with a paper crimper.