I love my Fiskars paper trimmer! There are several models by various companies which work essentially the same way as the trimmer pictured above. You place the paper on the trimming board and slide the blade down. That's it! I used to avoid buying large scrapbook paper just because I hated spending time trimming things down to size. This trimmer allows me to have straight cuts every time! I was able to get mine on sale at JoAnn for $12.99 (I think the retail cost was $19.99) and it has so been worth it! I cannot imagine working on any craft project without this tool.
If you're wondering, the arm on the top slides up so you can measure paper up to 12 inches across. Super compact and easy to use. The only negative is that sometimes you get a slightly fuzzy edge when you cut. Some papers cut really cleanly, while others don't. It hasn't been a big deal for me; I just make sure to hold the trimmer down really firmly when I cut. The little fuzz is not nearly as bad as wobbly scissor-cut lines.
I am in love with this tool...can't imagine crafting without it.