I've used iSeek, and I liked it quite a bit when it first came out. It takes up a fair amount of precious menubar real estate, though, and it ain't free.

AcidSearch is a better solution, I think. It's free, it resides in the Google search box of Safari, has quite a few preset options to search sites (eg, Amazon, Versiontracker) and use various search engines. The blurb about it on Versiontracker at

http://www.versiontracker.com/dyn/moreinfo/macosx/24092

claims you can search via multiple search engines simultaneously. You supposedly can also create your own access to search engines not already offered by AcidSearch, although I couldn't get AcidSearch to work with Bookfinder.com and some others.

Don't let the low version number fool you. AcidSearch is quite good. I also like Huevos--also free--which resides in your dock. I found it easier to use Huevos to create access to sites not offered (Bookfinder.com) than AcidSearch, but your mileage may vary.