EcoBusinessLink has a good survey of various carbon offset companies and organization with details around price, project types, offset types, and certifications. I decided to check out Carbonfund.org, because they have multiple certifications, are not for profit, and allow a choice of which project type to support (e.g. Efficiency & Carbon Credits vs. Renewables & Methane).
The process was surprisingly straightforward. You simply pick a calculator (I picked "Flight," because I want to offset my upcoming flight to Sweden), enter airport information, click calculate, and add to cart:
My carbon offset price ended up at $15.22. I chose to funnel that money to Reforestation & Sequestration. I like trees. It took approximately three minutes to complete the entire order.