Founders are people who have purchased special packages that include a special forum emblem and platinum (with bonus) and may include special "Prime" equipment and more. Packages can be upgraded once purchased.
The founders program can be seen here on the official Warframe web site.
The founders program is inevitably going to end. There is no current date set; it was supposed to end before open beta, but was extended, partly to allow the new users brought in by the game's Steam release to be able to buy it, and also to give the developers more time to "nail down an incredible follow up offer to the Founders program", according to an administrator.
There are 4 packages.
Disciple is the lowest level at only $20 USD. It comes with the fewest bonuses.
Hunter is the next tier at $50 USD and comes with a substantial Platinum bonus. It is also the lowest level to gain beta keys and a Prime item, Excalibur Prime.
Master is the third tier for $100 USD, and comes with enough plat to purchase all warframes and slots for them. The master is the first package to come with the privilege to view the design council forums and vote on features for the game.
Grand Master for $250 USD is the final tier and comes with enough platinum to purchase nearly everything in the game that can be bought with platinum. This package also includes all the Prime equipment and any player with this level has his or her name placed in the game as a Solar Landmark.
Each of the packages come with platinum. The ratio of dollars to platinum is much better than paying for just platinum.
Badges are seen in the forums under player pictures to indicate the players founder package.
Prime equipment includes an Excalibur Prime, Skana Prime, and Lato Prime. They have slightly adjusted statistics from their non-prime counter parts, and special skins that stand out from the design of the rest of the game.
Beta keys are given to 3 of the tiers for the player to give to other potential players to participate in the closed beta program.
Design Council privileges are given to the top two tiers. This includes the ability to view design council forums (hidden to other players) and vote on designs, features, and names in the game. The staff is significantly more involved in the design council forums. Many announcements about the game are relesed there before announced to the free community.
The Solar Landmark is a descriptor of every mission in the game. It is displayed in the lower right of the info box of planets when the mouse is hovered over any mission node. The symbol is that of the Grand Master and the text is the user name of a Grand Master. With 210 missions (as of 3-15-2013) and every mission having a Solar Landmark, $52,500 USD has been donated from the Grand Master program alone. It is estimated that this represents less than half the Grand Masters.
Once a player has purchased one of the lower 3 packages, he or she can later upgrade to one of the other higher tiers for the difference of the price and earn the difference of the rewards.