I have read that certain iRobot products support or can be hacked to support something close to the open interace. There is even a book about hacking Roomba. What Robots have this capability?
Any iRobot product with a 7-pin mini-DIN connector has some subset of the Open Interface. Roomba 500 through 800 series should correspond to the set in the Create 2 manual while the 400 or Discovery series correspond to the original iRobot Create. However, based on the date of manufacture, it may be necessary to apply a software update to the robot in order to enable this functionality.
Please note that I am an iRobot employee, but the postings on this site are my own and don't necessarily represent iRobot's positions, strategies, or opinions.
The Roomba Open Interface has been standard on all models since 2005.
Hacking Roomba uses the third generation Roomba Red for many of it's example hacks. The original Create and those early Open Interface capable Roomba's, have their serial ports above the power socket, beneath a removable cover:
More recent Roombas, from 500 series onwards, have the serial port in the same position as the Create 2, but hidden under the top cover:
Wired suggested that the newest Wi-Fi equipped Roomba 980, as the first which is controllable via an official smartphone (Android & iPhone), may not be hacker friendly - implying that it doesn't have an Open Interface, but OI transcripts have been posted, suggesting that it is still possible.