In terms of software that uses object points as an input, I've never run across any. I've read about tools that use function points, and I've either seen or used SLIM and USC COCOMO II, and neither of them appeared to allow for object-points as an input. Both of these tools used KLOC as an input - if you're able to estimate KLOC based on object-points, these would work.
As for free software, USC COCOMO II is free for personal or educational use and is available from the COCOMO II website. The last release was in 2000 and it runs on Windows 95, 98, NT, and XP (direct link to download page). Also using the COCOMO tools, there is a COCOMO Suite of Constructive Cost Models and a COCOMO 81 calculator online.
Most of the software is commercially available, though. SEER-SEM and SLIM are fairly popular. It looks like they have demos, available, so you can try before you buy.