From MongoDB's webpage I understand that they are not supporting transactions fully, if any.
I wonder if they are ever going to support it in the future so that I can store financial information in them, instead of using a RDBMS for it.
And how is it with CouchDB, do they support transactions?
Neither of these supports transactions in the sense of the more traditional RDMS - and it's unlikely they will - it's a tradeoff, supporting transactions in a distributed system is non-trivial and expensive.
MongoDB does not have ACID properties, and likely never will. CouchDB does give you ACID (I'm not sure if it does by default).
Both allows you to perform simple atomic operations on data, such as simple add/subtract on values though.
See also
http://stackoverflow.com/questions/299723/can-i-do-transactions-and-locks-in-couchdb
http://stackoverflow.com/questions/2655251/mongodb-transactions
On that note, this podcast with one of the MongoDB guys should give you an brief overview of the problems many NoSQL systems tries to solve, and the tradeoff they make.