They're considered the most precious of all domain commodities: 1-character names.
But they've been unavailable for years in all the major generic extensions (GTLDs). In fact, all 16 gTLD registries are obligated under the terms of their contracts with ICANN to reserve single character names so that they may not be registered.
But there are plans afoot to change that. Last year, ICANN's Generic Names Supporting Organization (GNSO) recommended that 1 and 2 character domains be released in existing and future gTLDs. ICANN itself controls all but 6 of the possible 144 single character names (letters and numbers) in .COM, .NET, .EDU and .ORG (the exceptions are q.com, x.com, z.com, i.net, q.net, and x.org) and is known to be considering auctioning off the single character .COMs.
Can't say I'm a big fan of registry-run auctions, or auctions run by others with a monopoly over the resource they're selling, which is why I was encouraged to read .MOBI and .COOP's May 29th registry requests to ICANN for the release of single character (and 2 character in .COOP's case) domains.