For now the most preferable infrared leds, that most of the people use, are the Osrams SFH485 or SFH485P. Specifications: 1.5 V 100(mA) 880 (nm) 5mm. The difference beetween the SFH485 and SFH485P is that the first one has a beam angle of 40°. The “P” version has a wider beam angle of 80° and comes with a flat head:

Order here for example.

Apparently, the above link no longer sells IR Leds of any kind (that I could find), so here is the same LED from Digi-Key

In doing research for the ideal LED, I came across the Osrams SFH-4350, which are identical to the 485’s, but 3mm instead of 5mm, and are about 20% cheaper.

You can order them from Digi-Key also.

NOTE: I just compared prices for the 5mm and 3mm. The cost for 100 3mm is $43.50 compared to the $38 cost of 100 5mm. It is cheaper to buy the 5mm as of this time (2/11/08) from digikey.com.
-O’Malley