The following table sets forth the location, approximate square footage and primary use of each of the principal properties we occupied at December 31, 2008:
The following table shows certain selected quarterly metrics for 2007 and 2008:
2007 compared to 2006-Commission revenue increased $22.6 million, or 38%, in 2007 compared to 2006, primarily due to an increase in our membership. Our estimated membership increased approximately 32% to 518,400 at December 31, 2007 from 393,900 at December 31, 2006. Included in commission revenue for 2006 and 2007 was $0.5 million and $0.7 million of commission revenue, respectively, we recognized from certain small business members who were transferred to us by a partner with whom we share a percentage of the ongoing commissions we receive on these transferred policies. Commission revenue for 2006 also included the recognition of $0.5 million of revenue related to commissions received from a single health insurance carrier during 2005, which was included in deferred revenue at December 31, 2005. Sponsorship, licensing and other revenue increased $3.9 million, or 166%, in 2007 compared to 2006, primarily due to increased sales of carrier sponsorship advertising on our website and, to a lesser extent, new licensing arrangements related to our technology.
2007-In the fourth quarter of 2007, we concluded, based upon recent operating results, expectations of future taxable income, available carryforward periods and other factors, that it was more likely than not that we will realize sufficient earnings to utilize all of our deferred tax assets. Accordingly, we reversed the remaining valuation allowance against deferred tax assets and recorded a tax benefit of $18.9 million. This benefit was partially offset by a provision for income taxes of $8.6 million for 2007.
judgment is required in determining the point at which various projects enter the states at which costs may be capitalized, in assessing the ongoing value of the capitalized costs and in determining the estimated useful lives over which the costs are amortized, which is generally three years. To the extent that we change the manner in which we develop and test new features and functionalities related to our website, assess the ongoing value of capitalized assets or determine the estimated useful lives over which the costs are amortized, the amount of website development costs we capitalize and amortize in future periods would be impacted. Through December 31, 2008, a majority of our internal-use software and website development costs have been expensed as incurred.
We adopted SFAS No. 159 (“SFAS 159”), The Fair Value Option for Financial Assets and Financial Liabilities, effective January 1, 2008. SFAS 159 allows an entity the irrevocable option to elect fair value for the initial and subsequent measurement for specified financial assets and liabilities on a contract-by-contract basis. The implementation of SFAS 159 did not have an impact on our consolidated financial position, results of operations or cash flows, as we did not elect to apply the fair value option under this Statement.
The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of our common stock at December 31, 2008. Total intrinsic value of stock options exercised during the years ended December 31, 2006, 2007 and 2008 was $1.9 million, $62.0 million and $7.9 million, respectively.
At December 31, 2008, we had tax credit carryforwards of approximately $1.9 million and $1.4 million for U.S. federal income tax purposes and state income tax purposes, respectively. Federal tax credit carryforwards begin expiring in 2019 and state tax credits carry forward indefinitely.
Leases-We lease certain of our office and operating facilities and certain furniture and fixtures under various operating leases, the latest of which expires in December 2012. In addition, we lease office equipment under operating leases that range in original terms from three to over five years, the latest of which expires in July 2012. Certain of these leases have free or escalating rent payment provisions. We recognize rent expense on our operating leases on a straight-line basis over the terms of the leases, although actual cash payment obligations under certain of these agreements fluctuate over the terms of the agreements. Total rent expense under all operating leases was approximately $1.9 million, $2.4 million and $3.3 million for the years ended December 31, 2006, 2007 and 2008, respectively.