Imagine one of the following scenarios: you're reading a paper on your computer and want to look up a reference, or someone emails you the citation to an interesting paper, or you see some paper referenced on a website. If you want to read the paper that corresponds to the citation, you need to go to the website of the journal publisher, and enter all of the citation details or else click your way through years and journal issues to get to the paper you want. This is just the sort of tedious task that your computer should be doing for you instead.
And now, thanks to Geoffrey Bilder of CrossRef, it can. The details are described on the CrossTech blog. What it boils down to is that you can bring up the Ubiquity command window (CTRL+SPACE on Windows once Ubiquity is installed on Firefox), type "crossref" and use CTRL+V to paste a citation (or else select a citation on the webpage). After a couple of seconds, you'll see several hits for the paper in the CrossRef database, and clicking on Resolve will bring you directly to the online version of the paper. If you're feeling lucky you can just hit Enter after entering the citation and go directly to the first hit.
If you haven't already installed Ubiquity (see here for more info), I think it is worth it just to be able to use this particular CrossRef script.