dvd's comment about favelets yesterday reminded what fabulous things they are. The favelets outlining and labelling divs at Accessify are genuinely useful, and can highlight problems that you could fail to find after hours of staring at the code. A couple of days ago, after Zeldman mentioned it, I checked out the Ruler favelet at Centricle which allows you to select regions of a page and see a very neat box giving the pixel dimensions and positions of the selection. I don't know about you, but I haven't done enough web design yet to intuitively know what 100 pixels look like.
The prize for most useful favelet of the day goes to Autofill (found via Macosxhints -- it gives you the convenience of autofilled forms with any browser. Given that autofill was the only thing I liked about IE, this is fantastic.
(A note for Safari users: trying to drag favelets onto the bookmark bar will only result in an embarrassing crash: you have to control-click/right-click the link and choose "Add to bookmarks".)