On April 10th 2012 IndexTank will shut down their servers.
I suggest to build a list of IndexTank compatible API companies.
Open Source Version: https://github.com/linkedin/indextank-engine
Searchify: http://www.searchify.com/ (Running the full open sourced IndexTank Search as a Service API)
IndexDen: http://indexden.com/ (IndexTank Compatible API - builded on top of Sphinx Search)
HoundSleuth: http://www.houndsleuth.com/ (IndexTank Compatible API)
IndexTanktoGO: http://launch.indextanktogo.com/ (IndexTank Compatible API)
Probably some more companies should be mentioned here.