Currently the cheapest I'm aware of are:
* .com - Fabulous.com ($7.30/year); United-Domains.de (1 Euro promo for 1st year), then transfer to GoDaddy 2+ months before expiration ($6.99 .com transfers)
* .net - Fabulous.com ($5.35/year); Name.com ($5.99/year)
* .org - Fabulous.com ($6.65/year); Dynadot.com ($6.99/year)
DomainCentral.com used to be $5 for the first year for all 3 of the above, but they recently jacked that figure up to $8.
DomainsAreFree.com claims to offer all 3 TLDs for $3.99/year, but I personally wouldn't touch them with a fifty-foot pole.