I assume by "make it appear brighter", you are thinking of focusing the beam? Realise that sometimes the side spill is just as important to the overall useability of the light as the main beam. Also realise that nothing is perfect. If you put a lens in front of your light source, you may focus the beam, but you'll also have about 15% loss in overall output. Ditto for reflectors.
Now before we go any further, can you say which type of LED you were looking at?
Luxeons have lots of lenses and reflectors for it. The NX-05, FHS lenses and IMS reflectors (check out Dat2Zip's and Elektrolumen's sites). There is also the Carlco Optics stuff, but I haven't found a retailer yet.
5mm, 8mm and 10mm LEDs already have integral lenses that focus the output down to 15deg so there are many lenses available for them. Also, the output is comparatively weak, so popping an additional lens in front of some of them can make them unviable.
And there are a couple more LEDs, like the Superflux Spiders and surface mounted.
So which were you looking at?
