As Bruce Miller points out, the Documentation Center is the key place to go generally to drill down for information on what's in the Wolfram Language. It is a huge resource and works pretty well in finding content listings that are relevant. Once you are on any given page you can spider out from there from the contained links and resources. It's what I constantly do as I write code since I do not remember (nor am I at all up to date) with what's in the system. But I remind myself what things that I know that I know by just diving into the documentation center.

That said, you may have something else in mind. Perhaps you are wanting to create a targeted resource to a particular field--for example organic compounds. Though you could create a user interface to do this using Entities and ChemistryData and so on, similar to the Google Instant approach, it might be very slow since generally each query as you type needs to reach out to the cloud to gather relevant content. A solution to this might be to download the relevant namespace materials and build the application based on a local data set of these...

This will give you a list of all organic compound entities that Mathematica has available (warning: it is a long list!):

EntityList[EntityClass["Chemical", "Organic"]]

rather than execute that and fill up your screen, here are properties that you might access for these:

EntityValue[EntityClass["Chemical", "Organic"], "Properties"]

which gives

{EntityProperty["Chemical", "AcidityConstants"], 
 EntityProperty["Chemical", "AdjacencyMatrix"], 
 EntityProperty["Chemical", "AlternateNames"], 
 EntityProperty["Chemical", "AtomPositions"], 
 EntityProperty["Chemical", "AutoignitionPoint"], 
 EntityProperty["Chemical", "BeilsteinNumber"], 
 EntityProperty["Chemical", "BlackStructureDiagram"], 
 EntityProperty["Chemical", "BoilingPoint"], 
 EntityProperty["Chemical", "BondCounts"], 
 EntityProperty["Chemical", "BondEnergies"], 
 EntityProperty["Chemical", "BondLengths"], 
 EntityProperty["Chemical", "CASNumber"], 
 EntityProperty["Chemical", "CHBlackStructureDiagram"], 
 EntityProperty["Chemical", "CHColorStructureDiagram"], 
 EntityProperty["Chemical", "CIDNumber"], 
 EntityProperty["Chemical", "Codons"], 
 EntityProperty["Chemical", "ColorStructureDiagram"], 
 EntityProperty["Chemical", "CombustionHeat"], 
 EntityProperty["Chemical", "CriticalPressure"], 
 EntityProperty["Chemical", "CriticalTemperature"], 
 EntityProperty["Chemical", "Density"], 
 EntityProperty["Chemical", "DielectricConstant"], 
 EntityProperty["Chemical", "DipoleMoment"], 
 EntityProperty["Chemical", "DOTHazardClass"], 
 EntityProperty["Chemical", "DOTNumbers"], 
 EntityProperty["Chemical", "EdgeRules"], 
 EntityProperty["Chemical", "EdgeTypes"], 
 EntityProperty["Chemical", "EGECNumber"], 
 EntityProperty["Chemical", "ElectronAffinity"], 
 EntityProperty["Chemical", "ElementCounts"], 
 EntityProperty["Chemical", "ElementMassFraction"], 
 EntityProperty["Chemical", "ElementTypes"], 
 EntityProperty["Chemical", "EUNumber"], 
 EntityProperty["Chemical", "FlashPoint"], 
 EntityProperty["Chemical", "FormalCharges"], 
 EntityProperty["Chemical", "FormattedName"], 
 EntityProperty["Chemical", "Formula"], 
 EntityProperty["Chemical", "FormulaString"], 
 EntityProperty["Chemical", "FusionHeat"], 
 EntityProperty["Chemical", "GmelinNumber"], 
 EntityProperty["Chemical", "HBondAcceptorCount"], 
 EntityProperty["Chemical", "HBondDonorCount"], 
 EntityProperty["Chemical", "HenryLawConstant"], 
 EntityProperty["Chemical", "HildebrandSolubility"], 
 EntityProperty["Chemical", "HillFormula"], 
 EntityProperty["Chemical", "HillFormulaString"], 
 EntityProperty["Chemical", "InChI"], 
 EntityProperty["Chemical", "IonCounts"], 
 EntityProperty["Chemical", "IonEquivalents"], 
 EntityProperty["Chemical", "Ions"], 
 EntityProperty["Chemical", "IsoelectricPoint"], 
 EntityProperty["Chemical", "IsomericSMILES"], 
 EntityProperty["Chemical", "IUPACName"], 
 EntityProperty["Chemical", "LewisDotStructureDiagram"], 
 EntityProperty["Chemical", "LightSpeed"], 
 EntityProperty["Chemical", "LogAcidityConstants"], 
 EntityProperty["Chemical", "LowerExplosiveLimit"], 
 EntityProperty["Chemical", "MDLNumber"], 
 EntityProperty["Chemical", "MeanFreePath"], 
 EntityProperty["Chemical", "MeltingPoint"], 
 EntityProperty["Chemical", "Memberships"], 
 EntityProperty["Chemical", "MolarMass"], 
 EntityProperty["Chemical", "MolarVolume"], 
 EntityProperty["Chemical", "MolecularMass"], 
 EntityProperty["Chemical", "MoleculePlot"], 
 EntityProperty["Chemical", "Name"], 
 EntityProperty["Chemical", "NetCharge"], 
 EntityProperty["Chemical", "NFPAFireRating"], 
 EntityProperty["Chemical", "NFPAHazards"], 
 EntityProperty["Chemical", "NFPAHealthRating"], 
 EntityProperty["Chemical", "NFPALabel"], 
 EntityProperty["Chemical", "NFPAReactivityRating"], 
 EntityProperty["Chemical", "NonHydrogenCount"], 
 EntityProperty["Chemical", "NonStandardIsotopeCount"], 
 EntityProperty["Chemical", "NonStandardIsotopeCounts"], 
 EntityProperty["Chemical", "NonStandardIsotopeNumbers"], 
 EntityProperty["Chemical", "NSCNumber"], 
 EntityProperty["Chemical", "OdorThreshold"], 
 EntityProperty["Chemical", "OdorType"], 
 EntityProperty["Chemical", "PartitionCoefficient"], 
 EntityProperty["Chemical", "pH"], 
 EntityProperty["Chemical", "Phase"], 
 EntityProperty["Chemical", "ProtonAffinity"], 
 EntityProperty["Chemical", "RefractiveIndex"], 
 EntityProperty["Chemical", "RelativeMolecularMass"], 
 EntityProperty["Chemical", "Resistivity"], 
 EntityProperty["Chemical", "RotatableBondCount"], 
 EntityProperty["Chemical", "RTECSClasses"], 
 EntityProperty["Chemical", "RTECSNumber"], 
 EntityProperty["Chemical", "SideChainLogAcidityConstant"], 
 EntityProperty["Chemical", "SMILES"], 
 EntityProperty["Chemical", "Solubility"], 
 EntityProperty["Chemical", "SpaceFillingMoleculePlot"], 
 EntityProperty["Chemical", "StandardName"], 
 EntityProperty["Chemical", "SurfaceTension"], 
 EntityProperty["Chemical", "TautomerCount"], 
 EntityProperty["Chemical", "ThermalConductivity"], 
 EntityProperty["Chemical", "TopologicalPolarSurfaceArea"], 
 EntityProperty["Chemical", "UpperExplosiveLimit"], 
 EntityProperty["Chemical", "VanDerWaalsConstants"], 
 EntityProperty["Chemical", "VaporDensity"], 
 EntityProperty["Chemical", "VaporizationHeat"], 
 EntityProperty["Chemical", "VaporPressure"], 
 EntityProperty["Chemical", "VertexCoordinates"], 
 EntityProperty["Chemical", "VertexTypes"], 
 EntityProperty["Chemical", "Viscosity"]}

Here are the first few of the (huge number of) organics that are returned by the

someOrganics={Entity["Chemical", "1,3Propanediol(2)13C1"], 
 Entity["Chemical", "1,4BisTrifluoromethylbenzene13C6"], 
 Entity["Chemical", 
  "1R,2RMinusPseudoephedrineNMethylHydrochlorideD3"], 
 Entity["Chemical", "2Butoxyethanol(O)D1"], 
 Entity["Chemical", "2Chlorodiphenyl(2,3,4,5,6)D5"], 
 Entity["Chemical", "4FluorobenzoylChloride(Carbonyl)13C1"], 
 Entity["Chemical", "4HydroxybenzoicAcid13C1"], 
 Entity["Chemical", "5Fluorouracil15N2"], 
 Entity["Chemical", "Acetamide(2)D3"], 
 Entity["Chemical", "Benzene(1,2,3,5)D4"]};

And from these and various properties (not all of which are available for each given Entity) you can get information like this

coords= EntityValue[Entity["Chemical", "Benzene(1,2,3,5)D4"], EntityProperty["Chemical", "VertexCoordinates"]]

Which gives

{{340.3, 100.}, {253.69, 50.}, {426.9, 
  50.}, {253.69, -50.}, {426.9, -50.}, {340.3, -100.}, {340.3, 
  162.}, {200., 81.}, {480.59, 
  81.}, {200., -81.}, {480.59, -81.}, {340.3, -162.}}

And you can visualize this like this, for example,

Graphics[{AbsolutePointSize[10], Point[coords]}]

I hope this helps to get you started...