Adblock Plus and NoScript are essential for me. I tend to use a lot of very slow computers (embedded and such) and with some websites throwing Flash and Javascript around all willy-nilly it can prevent me from getting my work done. Those two extensions put me back in control of my browsing experience.
Some lambaste ABP as being unfair for sites that depend on ad revenue to stay afloat but as I see it I wasn't going to purposefully click on any ads and I definitely don't want to pay any respect to anyone involved in those irritating flashing/noisemaking/take-over-your-whole-screen-until-you-locate-the-tiny-close-button ads.