Adblock - Exactly what it sounds like. Incidentally, it makes picking up printables (coloring pages, crafts, etc.) from Nick Jr., Disney Magic Artist Online, and the like go much faster.
AniDisable - Adds control for .gif animation (normal, once, or none) to the context menu.
BugMeNot - Allows you to bypass the login of websites that require compulsory free registration and/or the collection of personal/demographic information (such as the New York Times).
ChromEdit - A simple user profile file editor.
ColorZilla - Graphics tools: eyedropper, color picker, page zoom, DOM inspector.
downTHEMall! - A download tool that filters the extensions of all the links on a page, then allows you to select which to download with a single click.
Flashblock - Replaces Flash objects with a button you can click to view them.
ForecastFox - Gets weather forecasts from weather.com and displays them in any toolbar or status bar. Totally pwns waiting for the local weather news.
Gmail Notifier - Exactly what it sounds like. Adds an unobtrusive icon to the status bar.
ieview - Adds "View page in Internet Explorer" links to the content and link context menu.
MapIt! - Highlight an address and get a map and/or driving directions using your favorite online mapping site.
miniT (drag+indicator) - Adds tab dragging with drop place indicator.
OpenBook - Allows customization of the Add Bookmark dialog.
Paste and Go - Exactly what it sounds like. I missed this feature after switching from Opera.
SessionSaver - Auto-tracks, saves and restores window positions, tab histories, cookies and scrolling on startup.
Show Image - Adds "Show Image" to context menu entry to reload images.
Slim Extension List - hides descriptions and shrinks icons in extension list except for currently active extension (i.e., clicked on) and sorts them by name.
Stop-or-Reload Button - combines the stop and reload buttons, displaying the stop button while a page is loading and the reload button the rest of the time.
Tabbrowser Preferences - Enables enhanced control for tabbed browsing.
ViewSourceWith - Allows you to view page source with an external editor.
Wikipedia - Adds a new toolbar for easier editing of Wikipedia pages. This has tempted me to migrate TODCRA to a wiki, but I haven't brought it up for discussion yet.
I also use these search engine plugins: AllRecipes, Amazon.com, eBay, freedb (a GPL CD database server), Google, IMDb, RecipeSource, Urban Dictionary, Wikipedia, Wordorigins. (If you're wondering about the lack of a regular dictionary search in there, I use the Google "define:" boolean before more specialized dictionary searches.)
I'd like to see search plugins for the Merck Manual, RxList, and