Alternatives include: JML, C4J, Contract4J, JASS, and Modern Jass. The most popular and well-known solution is probably JML.
I'd like to mention though that Cofoja itself is not so "new", in a sense; it is a rewrite of Modern Jass, which itself was a rewrite of JASS. Though the code may be mostly new, we've borrowed many concepts, ideas, and implementation techniques from our predecessors, and hopefully learnt a bit from their experience.
One could then ask "why pick Modern Jass as a basis while there are other more <whatever> framework available?" but that's another question.
Links: C4J. http://c4j.sourceforge.net/ Contrac4J. http://www.contract4j.org/ Java with Assertions (JASS). http://csd.informatik.uni-oldenburg.de/~jass/ Java Modeling Language (JML). http://www.jmlspecs.org/ Modern Jass. http://modernjass.sourceforge.net/
And if you're interested in the fine details of the internal differences between Cofoja and other frameworks, you may want to read my technical report: http://cofoja.googlecode.com/files/cofoja-20110112.pdf