Only about 6 single letter and single digit domain names have been registered by a few lucky or ###### insiders. ICANN reserved all single letter and single digit second-level domain names in the TLDS .COM .NET .ORG back in 1993. This was done in case the registries for these domains became overloaded (smells fishy to me). The 6 domains that were already registered were not revoked and were exempt from this ruling and allowed to stay in use.

Source: Wikipedia

March 15/85 www.symbolics.com
April 24/85 www.bbn.com
May 24/85 www.think.com
July 11/85 www.mcc.com
September 30/85 www.dec.com
November 7/85 www.northrop.com
January 9/86 www.xerox.com
January 17/86 www.sri.com
March 3/86 www.hp.com
March 5/86 www.bellcore.com
March 19/86 www.ibm.com
March 19/86 www.sun.com
March 25/86 www.intel.com
March 25/86 www.ti.com
April 25/86 www.att.com
May 8/86 www.gmr.com
May 8/86 www.tek.com
July 10/86 www.fmc.com
July 10/86 www.ub.com
August 5/86 www.bell-atl.com
August 5/86 www.ge.com
August 5/86 www.grebyn.com
August 5/86 www.isc.com
August 5/86 www.nsc.com
August 5/86 www.stargate.com
September 2/86 www.boeing.com
September 18/86 www.itcorp.com
September 29/86 www.siemens.com
October 18/86 www.pyramid.com
October 27/86 www.alphadc.com
October 27/86 www.bdm.com
October 27/86 www.fluke.com
October 27/86 www.inmet.com
October 27/86 www.kesmai.com
October 27/86 www.mentor.com
October 27/86 www.nec.com
October 27/86 www.ray.com
October 27/86 www.rosemount.com
October 27/86 www.vortex.com
November 5/86 www.alcoa.com
November 5/86 www.gte.com
November 17/86 www.adobe.com
November 17/86 www.amd.com
November 17/86 www.das.com
November 17/86 www.data-io.com
November 17/86 www.octopus.com
November 17/86 www.portal.com
November 17/86 www.teltone.com
December 11/86 www.3com.com
December 11/86 www.amdahl.com
December 11/86 www.ccur.com
December 11/86 www.ci.com
December 11/86 www.convergent.com
December 11/86 www.dg.com
December 11/86 www.peregrine.com
December 11/86 www.quad.com
December 11/86 www.sq.com
December 11/86 www.tandy.com
December 11/86 www.tti.com
December 11/86 www.unisys.com
January 19/87 www.cgi.com
January 19/87 www.cts.com
January 19/87 www.spdcc.com
February 19/87 www.apple.com
March 4/87 www.nma.com
March 4/87 www.prime.com
April 4/87 www.philips.com
April 23/87 www.datacube.com
April 23/87 www.kai.com
April 23/87 www.tic.com
April 23/87 www.vine.com
April 30/87 www.ncr.com
May 14/87 www.cisco.com
May 14/87 www.rdl.com
May 20/87 www.slb.com
May 27/87 www.parcplace.com
May 27/87 www.utc.com
June 26/87 www.ide.com
July 9/87 www.trw.com
July 13/87 www.unipress.com
July 27/87 www.dupont.com
July 27/87 www.lockheed.com
July 28/87 www.rosetta.com
August 18/87 www.toad.com
August 31/87 www.quick.com
September 3/87 www.allied.com
September 3/87 www.dsc.com
September 3/87 www.sco.com
September 22/87 www.gene.com
September 22/87 www.kccs.com
September 22/87 www.spectra.com
September 22/87 www.wlk.com
September 30/87 www.mentat.com
October 14/87 www.wyse.com
November 2/87 www.cfg.com
November 9/87 www.marble.com
November 16/87 www.cayman.com
November 16/87 www.entity.com
November 24/87 www.ksr.com
November 30/87 www.nynexst.com

Source: Wikipedia