If you're looking for examples of implementations, greenmountainenergy.com has a good one, the EPA has calculator (and data tables for download), sterlingplanet.com has one, nativeenergy.com has one, carbonfund.org has one, terrapass.com is another. Some of the data they're using comes from the transportation energy data book (or digital versions thereof), WRI GHG calculations or ISO 14000 series environmental management methods.
The rest of the problem is the credibility of the verification of the demonstration of additionality of the offsets. Try to adhere to The Gold Standard.
Hopefully you knew most of that already.
Perhaps you could setup a social network where engineers doing research into making alternative energy viable post profiles. Users hoping to reduce their footprint could help support the research -- something like kiva.org for the first world.
Funding a few post-docs to make an efficient solar cell will matter more in the long run than all carbon offsets combined.
This means that a reasonable consultancy would always advocate simply spending money on such research.
Things like compact florescent light bulbs are becoming popular. Why? Because there is a cost savings in power over regular light bulbs, NOT because they are good for the environment. Reuse and reduction of unnecessary energy is a good thing. Money spent on alternatives that don't scale into a global solution is money wasted.
But I didn't actually answer your question: what is involved in building such a site?
Briefly, software engineers build up a back end in a language like Perl, PHP, Python, or Ruby. They are supported by databases like mysql or sqllite. They run on servers rented in a hosted configuration or purchased and installed at a co-location facility like an ISP. The front end involves HTML, javascript, and maybe some Flash. It also involves work to ensure the interaction design makes sense so people know how to use your site.
I answer so generally because your site isn't doing anything too difficult. Lots people could make it.
You'll want to hire people to get this done. Job boards like dice.com might be the way to go, but hiring good people is generally a hard problem.
"What we do in the next two to three years will determine our future. This is the defining moment." - Rajendra Pachauri, a scientist and economist who heads the IPCC
The paragraph from the report that I found most revealing was the following:
"Climate change is likely to lead to some irreversible impacts. There is medium confidence that approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average temperature increase exceed 1.5-2.5 degrees C (relative to 1980-1999). As global average temperature increase exceeds about 3.5 degrees C, model projections suggest significant extinctions (40-70% of species assessed) around the globe."
The IPCC's economic analyses say that trend can be reversed at reasonable cost. Indeed, it says, there is "much evidence that mitigation actions can result in near-term co-benefits (e.g. improved health due to reduced air pollution)" that may offset costs."
While I agree that research is necessary, it takes a long time. There are already a lot of grants and VC money going into greentech, but more money doesn't always correlate to faster progress (as all of you know).
Also, it will increase the supply of energy, and increase the demand of cheap carbon-producing energy to match. Total energy consumption will increase.
My point is not that climate change is unimportant.
My point is that carbon offsets that invest in the current generation of hopelessly expensive alternative energy approaches will NOT help the situation.
While you think research takes a long time, how long will a net harmful solution take to solve the problem?
Investing in research will help. It is the only long term solution.
I have looked into writing something like this before. In my case the technology was not difficult to do. The problem we had was regulation, auditing and the fact that no-one could agree which formulas to use. Different studies take different approaches and in our case it was a constant fight between scientists and the government. Banking these credits was also an issue.
If you have above 3 problems sorted out, writing something like this won't impossible.